It appears that I have acquired the actual Mellotron that was used on the 1974 recording session in which Freddy Fender re-recorded Wasted Days and Wasted Nights, which went on to be a big hit. It was owned and played by a man named Leo O'neil, who was actually a horn player. I tracked Leo down and he gave me some history. He says he also played it live quite a bit and told me the one night Patrick Moraz was in the audience at one of Leo's gigs. It was after a Moody Blues show and Patrick came out and caught his show. Afterward, Leo says that Patrick came up to him and complimented him on his playing style and on the sound he was getting out of it.
Leo attributes his skill on the Mellotron to the fact that as a horn player he was used to playing fewer notes and to holding them for brief periods. He said his playing style was, "to use only one or two fingers down low and the one finger an octave or more above, and to move around on the keyboard so as not to run the tape out."
So there you have it.
I only have to trace one more link and I'll know for sure that this is that machine.
It's a real mess, but it will get better.