Deputy Director insignia.
Deputy Director was a senior position in Starfleet's Department of Internal Affairs.
Luther Sloan (of Section 31) claimed to hold this position while he posed as an agent of Internal Affairs while aboard Deep Space 9 in 2374. (DS9: "Inquisition")
Background
Luther Sloan in the uniform of a "Deputy Director"
The sole appearance of "Deputy Director" was in "Inquisition" when the title was held by Luther Sloan. Sloan's insignia has never appeared in any other production but appears to be a type of rank between captain and rear admiral. Numerous fan internet sites and comic books have hailed Sloan's insignia as a reappearance of the rank fleet captain, although there is to date nothing in canon to support this.