On another forum I found a post made by Chip Madinger, who researched a ton for his definitive Lennon day-by-day book, "Lennonology: Strange Days Indeed". He says:
"[...]It is well documented how George Martin worked on the three-track tapes to create a hybrid of the 1964 and 1965 recordings. Copies of the raw tapes are easily found online. However, it is not as widely known that in mid-1971, the tapes were prepared for commercial release.
While researching for Strange Days Indeed, I discovered documentation that both the August 29th (Sunday) and August 30th (Monday) 1965 shows had been mixed and mastered for both stereo and Quad at Record Plant (Project #623) in July 1971. There is no paperwork regarding the recording of the 1964 performance.
Although it was not documented, it is very likely that it was Phil Spector behind the board and, one would suspect, that John Lennon participated as well. The reason for this being that they had mixed six of the ten tracks on John's Imagine LP in Quad just the day before. Capitol was getting in to the multi-channel market, and who could blame them for preparing albums by their biggest artists for holiday release.
In a single session, Quad mixes were prepared of both 1965 shows (in their entirety), after which a hybrid performance was created (which tracks were taken from which show was not annotated). The following session, both a Quad and a stereo (presumably a collapsed version of the Quad) LP master were prepared at 15 ips and taken away.
Considering the source material, one could argue that the resultant production would have been disappointing (just have a listen to the Quad Imagine LP). But how many of us would have suspected that such a recording even existed?"