Way back before "home video", how many thought that the "UFO" was actually part of the ST universe?
I think many believed it was some ship intended to appear on TOS, but somehow missed its big debut. In the late 60's the kit was called the "Leif Ericson" molded in a silver/gray tone, and was supplied with a cardboard record (45 rpm).
Some used to believe that due to AMT's relationship with Roddenberry/ST (see: the Galileo
), and the name "Leif Ericson" was used by Marla McGivers to describe Khan in "Space Seed"
meant the kit design was to be Khan's sleeper ship, but that's just conjecture--stringing a few random coincidences together.
What we do know is that the ship design was from the pen of Matt Jefferies, and years after TOS, the design was considered for use on the Filmation animated series--which would have been good for AMT, since they already had the existing kit to simply repackage with their ST graphics if the ship appeared on TAS.
Still, even without a direct association with TOS, the LE/UFO being AMT's only sci-fi themed kit outside of ST gave it the credential necessary to be lumped into the ST line (sort of similar to Aurora shoehorning its Ragnarok
air force plane into a 1975 reissue program with its sci-fi TV & movie-based kits).
I guess it did not matter, as many a kit builder or collector bought the "Leif Ericson" / "Interplanetary UFO" kit and set it right next to their U.S.S. Enterprise model (drooping nacelles included