The "Enterprise" was originally going to be named "Yorktown", but Roddenberry said he was fascinated by the story of the actual "Enterprise" and that he had "always been proud of that ship and wanted to use the name." The ship's "NCC-1701" registry stems from "NC" being one of the international aircraft registration codes assigned to the United States. The second "C" was added because Soviet aircraft used "C"s, and Jefferies believed a venture into space would be a joint operation by the United States and Russia. "NCC" is the Starfleet abbreviation for "Naval Construction Contract", comparable to what the U.S. Navy would call a hull number. Jefferies rejected "3", "6", "8", and "9" as "too easily confused" on screen; he eventually reasoned the "Enterprise" was the first vessel of Starfleet's 17th starship design, hence "1701". "The Making of Star Trek" explains that "USS" means "United Space Ship" and that ""Enterprise" is a member of the Starship Class". Licensed texts, on-screen graphics and props, and dialogue later describe the ship as a "Constitution"-class vessel.