L-374 was a planetary system. This system was located near the L-370 system and the Rigel system. The system was attacked by the "planet killer" in 2267. It had four terrestrial planets that were charted prior to the attack; the two outer planets were consumed as fuel by the machine before it could be destroyed by the starships USS Enterprise and USS Constellation. (TOS: "The Doomsday Machine")
Presumably, L-374 was located in the Alpha Quadrant.