Did a little reading on this - didn't know that several people overheard Zamora's radio traffic. I believe he saw something highly unusual that rattled him. Had to have been some terrestrial craft - clanking doors, roaring flames, two people standing next to it doesn't sound extraterrestrial to me. Project Gemini was in full swing and Apollo was gearing up. At that point the concept of the LEM existed but they hadn't worked out all the engineering. The final product was all about weight savings. Why the Air Force or NASA never owned up to it is strange but after more than a half century it could be a combination of secrecy at the time, bureaucracy, the routine destruction of old documents and death of the participants. Maybe it was a comparatively small project that wound up at a dead end and got overshadowed by the successful programs. The Bell Aerosystems Lunar Landing Research Vehicle (LLRV) - the 'flying bedstead' - absolutely existed in 1964 but hadn't officially flown when Zamora saw whatever it was. It was intended to give astronauts practice piloting the real lunar lander. Just to speculate maybe this was some earlier concept vehicle that wound up being ruled out - too heavy, whatever.