If you go to this link, you'll find a piece I put together around a film shot at Gold Star in early '66 which includes closeups of Stan Ross at the board:
http://www.classicstudiosessions.blogspot.com/2010/07/classic-session-at-gold-star-1966.htmlUnfortunately the camera didn't seem to get any shots of the outboard gear, just the board and the tape machines.
Compressors were used before the 60's obviously, and units like the RCA limiters were used in radio engineering and later taken out of that realm to be used in recording, whether or not they were used during mixing or tracking would be a subject for individual engineers and how they chose to work...now I do have a clip from the Headquarters Sessions where Mike is setting up his 12-string electric to record, and Hank Cicalo can be heard patching the track through a compressor/limiter and adjusting it as Mike is playing, which would suggest they did this for tracking with that specific 12-string "Byrds/Beatles" sound in mind as the track was recorded. And I believe, I could be wrong, but I *believe* they fed Roger McGuinn's 12-string through the chain of multiple compressors as he was tracking rather than tracking clean and then running through compression.
Interested to hear opinions on those. And also interested to hear about maxing out something like the LA-2A or the RCA in the 60's, we know Emerick did this with the Fairchilds in England but was anyone in the US doing that to this extreme prior to Revolver/Pepper? If anyone did I'd bet it would be Stan Ross and Larry Levine. They did it after bands and clients wanted their records to sound like the Beatles after Revolver...