I've been watching this auction, and wow ... that's a cool deck. But doesn't look like a machine that was put together by Ampex. The guy says it was built by the studio ... so I'm not sure how it could be called 'prototype'!
Also, I don't think 'Volunteers' was recorded at TTG. The MM1000 on the back of the album cover is different as well.
EDIT:
Maybe it's this machine?:
recordingsession : Ampex AG-1000 16 Track 2"
... which is weird because someone on that site writes that it was owned by the Grateful Dead.
Strange to me that there would be TWO custom jobs built in this manner, as opposed to the standard Ampex AG1000/MM1000 frame. The early ones would have been built by Ampex Special Products.
I'm skeptical regarding the history of the deck as well because some things don't add up. The electronics channels appear to be the 'B' versions, which were not introduced until 1969 ... yet some of records listed as recorded on the TTG 16-track were done in '68 ('Waiting for the Sun' for instance). Note the larger knobs -- the AG1000 from Mirasound has the 'A' version electronics with smaller knobs (w/ red tips on the repro knobs) ... the same ones as the AG350 and AG300.
Anything is possible ... but it seems funny to me.