Though only introduced in the third season of the Original Series, the Klingon D7-class battle cruiser, despite its limited number of appearances, has captured the imagination of the audiences, and went on to become one of the most signature ship designs of the franchise. Ironically, the original filming model was not commissioned by the producers for the show who had to contend with severe budget cuts during the third season, but resulted firstly from the wish of kit producer Aluminum Model Toys (AMT) to do a follow-up of their highly successful USS Enterprise model kit, as Jefferies himself revealed in Star Trek: The Magazine Volume 2, Issue 9 (page 66).
The D7-class studio model was originally designed by Matt Jefferies who needed about two months from start to final design sketch. Its design was explored in the Star Trek: The Original Series Sketchbook where it was explained that in Jeffries' attempt to create the D7, he "had to design a ship that would be instantly recognizable as an enemy ship, especially for a flash cut. There had to be no way it could be mistaken for our guys. It had to look threatening, even vicious." Taking an aerodynamic approach to his design, he ultimately "modeled it on a manta ray, both shape and color, and that's why it looks as it does in the original series." Jefferies later recalled on his design work:
"AMT didn't have any design input whatsoever, and by that point Gene pretty much left me on my own. I designed it here at home, because there was neither the time nor the money allowance to do it at the studio. Naturally, I thought it had to look as far out as we thought the Enterprise did. I was after a shape and didn't really know what the shape should be. I started doing little sketches, trying to come up with something; God knows how many there were. I saved some of them, but I'm sure I must have ashcanned maybe a hundred balled-up pieces of paper. It's like when you make a mistake in arithmetic and you go back over the same piece of paper and keep making the same dumb mistake; you've got to throw it away and start from scratch.(...)Having decided early on to make use of the same basic elements as with the Enterprise, twin nacelles and separate engineering and command hulls, with which he continued to experiment in layout and configuration, he added, "Sometimes if you feel you have something, which could be kind of rare, you turn it around in as many ways as possible, and all of a sudden something may pop up that makes more sense." The ship's design was perfected by the twenty-fourth sketch on 20 November 1967.
"The Klingons were supposed to a pretty wicked people, so I wanted something with a "killer potential" that would look wicked. Basically, I was feeding on the look of the stingray, or the manta ray, for part of the shape. Even though it is not dangerous, I think a lot of people think the manta ray has a very vicious look to it, yet when it swims it is very graceful. I was trying to get all of that in there. Then the coloration came directly from a shark, it's a grayish-green on top and a lighter gray underneath." (Star Trek: The Magazine Volume 2, Issue 9, pp. 66-69)
Jefferies sold off his original design sketches, as well as the below mentioned engineering drawings, on 12 December 2001 in the The Star Trek Auction, in order to raise funds for the charitable organization Motion Picture and Television Fund.
The original filming model
Jefferies proceeded to draw up detailed engineering drawings which also specified the scale in relation to the Enterprise, and took those to AMT. The scale drawings were also used as templates for the back-lit computer console readouts seen in the episode "The Enterprise Incident". Under Jefferies' own personal supervision, Gene Winfield's AMT-operated Custom & Speed Shop proceeded to manufacture two three-foot long "master tooling" models (more precisely, the dimensions were 28×20×7 inches ). A master tooling model served as a template for the molds from which AMT would cast the parts for their model kits. Jefferies recalled, "The master models were quite large; probably close to 18 inches across, I guess. They used what they call a pentagraph [sic]; at one end there was a stylus that traced its way over the master model, and at the other end there was a tool that carved out the same shape in tooling steel, which became the mold they built the kit from. [remark: the mold for the kits were pantographed at half-scale of the master model] I was there at about 2 o'clock in the morning when they ran the first two or three through the machine. They weren't perfect, so they said "We'll take out a fraction here, and a fraction here." Then they'd run two or three more. If I remember correctly, it was about 10 o'clock when the first one came out that they said was perfect. They ran maybe another half a dozen, and checked those out. One of which I still have; the box has never been opened. Then the machine was put in operation, and after that one came out every 20 seconds." (Star Trek: The Magazine Volume 2, Issue 9, pp. 69-70)
Under their exclusivity agreement the studio appropriated one of the master models for shooting purposes as they had done with the Galileo studio model. The model was subsequently turned over to the Howard Anderson Company for final detailing and filming stock footage for use in the show. In order to make maximum use of their new nifty model and as a courtesy to AMT (who after all paid for the model), in order to get them the most exposure for their new model kit (S952), the producers decided to use it wherever possible in the remainder of the third season of TOS and so the new model first appeared on-screen as a Romulan battle cruiser in "The Enterprise Incident", although it was first shot as a Klingon battle cruiser for "Elaan of Troyius" (which aired later). Its origin as a master tooling model for a model kit, as opposed to being an actual filming model was evident in the fact that neither model, both constructed out of solid wood, had internal lighting.
The original studio model was originally finished at the Howard Anderson Company in a multi-colored paint scheme (light green underneath and gray on top), applied personally by Jefferies. Jefferies also designed and applied the Klingon emblem and the Klingon lettering, the first time ever either one appeared in the Star Trek franchise. But the color scheme was obscured by studio lighting conditions, combined with the effect of lighting bouncing of the blue-screen onto the model, an effect known as "blue spill", resulting in what appeared to be a blue-gray overall color on screen. This impression was reinforced by the box art of the release of the D7 kit by AMT (No. S952). Interestingly, in the Animated Series episode "More Tribbles, More Troubles", Koloth's battle cruiser, IKS Gr'oth, adheres to the original paint scheme, though the two colors are reversed. Silver detail sections of the model were an adhesive Mylar.
After The Original Series was canceled, the original model of the D7 was, hand-delivered by Dorothy Fontana on 7 November 1973, donated to the Smithsonian National Air and Space Museum (NASM) along with the tiny "Catspaw" model of the USS Enterprise. "I was trying to figure out how to pack it up to ship it. Dorothy Fontana was headed back to D.C. and agreed to take it for me; we put it in a plastic garbage back, which was not deep enough to to take the whole thing, so the head of it stuck out. Somebody on the airliner recognized it, so they unwrapped it and it toured the airliner!", Jefferies recalled.(Star Trek: The Magazine Volume 2, Issue 9, pp. 69-70; Star Trek Giant Poster Book, issue 10, 1977)
Star Trek: Phase II derivatives
In the latest script treatments of the pilot episode "In Thy Image" of the projected Star Trek: Phase II television project, the Klingon-V'Ger encounter was already foreseen for that production. To this end, the original filming model was returned to Paramount Pictures in 1977 for intended use in the Phase II television production, and where it received a new paint job (gray, dark green and metal blue). It is conceivable that the new paint job was applied to repair damage, as the model was used as a master to take molds from in order to cast copies for intended use in the production. At the time, Jim Dow of Magician Inc., whose company was entrusted with the build of the studio models for Phase II recalled, "We began with the Klingon spaceship, as we had the 18-inch model from the original television series which had been loaned back to the production by the National Air and Space Museum. The basic form remaining similar, we began to make patterns and take measurements of the original, and a set of drawings was produced and blown up to eight feet then dropped back to four feet when [Robert] Abel [& Associates] decided they would never be able to get far enough away from the model to photograph it." (American Cinematographer, February 1980, p. 153) The outcome of the encounter was for this production originally storyboarded as ending with the destruction of the Klingon vessels. To this end breakaway models were constructed at Magicam for their on-screen destruction.
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According to the contemporary documentary, The Making of Star Trek: The Motion Picture, at least twelve breakaway models were constructed for the sequence, based on the molds taken from the original studio model. Footage of the destruction was shot at Abel's, but eventually not used, as it was quickly decided that the model would not stand up to the requirements television standards of the late 70s demanded, and especially not to the big screen requirements when Phase II was upgraded to a major movie project.
After the decision was made that three-foot models were not up to the task for the movie-upgrade, the original television studio model was of no further use, and so it was send back to the Smithsonian, were it sat in an unlit display case for the next ten or eleven years at the NASM's Garber restoration facility, not open to the public, before being disassembled and put away in storage. wbm It was still in its new paint scheme when Ed Miarecki undertook a major renovation on the model in 1991 in preparation for the Star Trek Smithsonian Exhibit and in 1993 the D7 model and the K't'inga-class model, which it had spawned, were reunited for a short time when the K'tinga was on loan to the Smithonian.  Miarecki, apparently unaware of the original paint scheme at the time, repainted the model in the blue-gray mono colored scheme as was normally perceived by television audiences.  Having only been displayed publicly once, the model, though in storage, is still in the possession of the Smithonian.
The second "master tooling" model
The second master tooling model was handed over to the studio a short time later after AMT was done with it. It received the same finishing touch as the first model and was at a casual glance nearly indistinguishable from the first one (exactly two times the size of the AMT model kit). There were, however, subtle differences between the two. The bulk of the differences were located on the engine nacelles. The position of the engine recessed side detail was moved more forward on the AMT prototype, in relation to the pylon foil vent. The detail piece on the engine bottom was a little further back on the studio model – about a third of its length back. There was a raised strip intersecting the chrome part of the side engine on the AMT version. The chrome part on the AMT prototype was a single strip shape that had a half-round profile view. The screen-used model version had two sets of two raised, flat chrome strips (AMT would correct this on re-issues of their model kit, as it later featured the two strips). And finally, the AMT model lacked both the feature in the torpedo launcher mouth, as well as the grills in the recessed forward "wing intakes" that were present on the screen-used model.
Although slated to be also used for filming it was never used as such and ended up in Roddenberry's office for awhile. Roddenberry gave the model away to Stephen Edward Poe in recognition of his help in establishing the co-operation between the Desilu studios and AMT (in whose employ Poe was at the time). It changed hands several times after that and was between 1998 and 2006 offered up for auction no less than four times. The first time it appeared, it was offered up for auction by Poe at Christie's Film and Television auction of 18 June 1998 as Lot 71 with an estimate of US$15,000–$20,000, selling for US$11,500 (including buyer's premium),  and was immediately offered up for auction afterwards as Lot 245 in Profiles in History's Hollywood Memorabilia Auction 5 on 12 December 1998, selling for US$35,000. The third time it appeared at auction was on 31 March 2003 in Profiles' Hollywood Auction 18 with an estimate of US$60,000–$80,000,  selling for US$55,000 ($64,900 including buyer's premium), before finally auctioned off as Lot 311 in Profiles' Hollywood Auction 24 on 31 March 2006 for US$65,000, estimated at US$65,000–$85,000.    It should be noted that all auction descriptions contained inaccuracies, and erroneously claimed that it was a screen-used model. The model was on the last occasion acquired by Microsoft's co-founder Paul Allen for his Science Fiction Museum and Hall of Fame in Seattle, where it currently resides. The deal was brokered before auction start and the model was therefore not featured in the auction itself.
The Phase II model
When the Phase II project was upgraded to a major movie project, what ultimately was to become Star Trek: The Motion Picture, the decision was made to have Magicam, no doubt due to their by then familiarity with the design of the ship, and supervised by Dow, built a new, more detailed twice as large scale model of the D7, based on the molds taken from the by now-disassembled original television model. Chris Ross was the lead modeler working on the model. The most noticeable difference from the original filming model, aside from its size, was the application of subtle hull paneling, reminiscent of the model Greg Jein later built for Star Trek: Deep Space Nine. Though commonly known as the "6-foot model", its exact measurements were 47.5×35 inches.  Construction on this model,  outfitted with an internal lighting system, and subsequent test shooting were in progress and signed off on when Magicam delivered the model to the producers in July 1978. However, further refinement for movie purposes by that time was deemed necessary, which was done at John Dykstra's Apogee, Inc. model shop, resulting in this model eventually becoming the K't'inga-class studio model. "We had to modify it pretty severely though, because apparently it was not designed to be shot in the same kind of circumstances that we ended up shooting it in. I don't know what Bob Abel planned to do with it, but for us, the practical lighting on the model was so dim that we weren't able to get a good exposure off it even by pushing the film a couple stops and using a twenty-second exposure", Dykstra elaborated. (Cinefex, issue 2, p. 52)
The Trials and Tribble-ations model
In 1996, a new model of the D7-class, IKS Gr'oth, was created by Greg Jein for Star Trek: Deep Space Nine's episode "Trials and Tribble-ations". While referenced, but not seen, in the original version of the "Trouble with Tribbles," the model featured an amalgam of detail from both the Original Series model and the K't'inga-class seen in Star Trek: The Motion Picture. The newer model bore a pale green coloration, in line with future Klingon vessels. Jein, a passionate life long fan of the original series, owned a set of molds of the original D7 studio model, made for Star Trek: Phase II , which he acquired after castings were made, and from those cast a model, which was further embellished. "Greg had a mold of the original, which had been on loan from the Smithsonian during the making of ST: TMP. The "Trials" ship was from that mold and uprezzed.", Doug Drexler later confirmed. wbm Apart from the additional slight hull detailing, reminiscent of the Phase II model, this model also sported internal lighting, strobe lights, and lights on the crown of the bridge. Although the producers originally did not want to have the model built, because of budget concerns, effects supervisor for the episode Gary Hutzel pushed the construction through, after he discussed the matter with Jein: "We talked for about fifteen seconds, and then Greg said "Oh, I'll build a shell. You paint it and detail it and we'll put it in the show."", adding "(We) took a little liberty. The original model had no lights on it at all, and it was pretty smooth, with no detail. We added neon for inside, strobe lights and some some lights on the crown of the bridge." (The Magic of Tribbles: The Making of Trials and Tribble-ations, pp. 45-46) Jein's passion for the project however, resulted in a full-blown studio model, produced at a bargain. Jein himself reiterated, "And I said, "You know, in that episode they had not the budget to show the Klingon ship", because a Klingon ship did not show up until the third season, this was a second season show, and we said how about we build a Klingon ship, and he [Hutzel] said, "No, we cannot definitely afford that, we're in trouble as it is!" So I said, "Tell you what, If the day comes when you need to shoot it, give us a call, and if you shoot it, fine, it's yours." So, we actually built the Klingon ship, which was Gary's heartset, so "the hell with it, we're gonna use it!", so he stuck it in the film." (Sense of Scale, disc 2)
The molds Jein owned were put to good use besides the IKS Gr'oth, apart from this ship he cast from the molds a D7-class display model that was featured in The Art of Star Trek, page 19 (the model was later given to Drexler, who auctioned it off as Lot 13 in the August 8th, 2010 Propworx Star Trek Prop and Costume Auction, estimated at US$2,000–$3,000, where it sold for US$12,000), as well as a commercial limited production run of twelve, without internal lighting, that was sold in 1997, accompanied with a certificate of authenticity signed by Greg Jein at the Viacom Entertainment Store in Chicago. wbm In addition to these, two K't'inga-class vessels were constructed from the molds: the unnamed K't'inga-class in TNG: "Unification I" and the IKS B'Moth in DS9: "Soldiers of the Empire".
The CGI model
Yet another model of the Gr'oth, this time built entirely digitally by CBS Digital and supervised by Dave Rossi and Niel Wray, was created for the 2006 "remastered" version of the various TOS episodes the design made an appearance in. In regards to this CGI model, when making those appearances, Michael Okuda noted that "the Klingon ship was basically in two forms. In early episodes, when it was very small on the screen, it was the original version of the ship, which had essentially no surface detail. In "The Enterprise Incident", "Elaan of Troyius", and "Day of the Dove", the ship was reworked somewhat to add surface texture. And, of course, in "The Enterprise Incident" we added the Romulan bird markings." 
Another CGI model was previously constructed by Doug Drexler for use in James Cawley's 2004 non-profit Internet fan series Star Trek: New Voyages. Nevertheless, that model also showed up in licensed publications, most notably the Star Trek: Ships of the Line calendars and their book derivative. Noteworthy, however, is that Drexler apparently had only an original AMT kit, or depictions thereof, at his disposal as reference, as his model mirrors the original second "master tooling" model, i.e., the single strip on the warp engines.
- "Behind the Scenes; Designing the Klingon Battle Cruiser", Star Trek: The Magazine Volume 2, Issue 9, January 2002, pp. 66-71