Bill Porter, legendary recording engineer out of Nashville, recorded *many* tracks at RCA Studio B with Chet Atkins (and others) as producer(s).
Bill's console had no pan pot. The inputs were switched left or right, 1 or 2. Therefore no center. I am sure other people were in that same situation. Even here in St. Louis, at late as 1968 there wasn't a recording console with a pan pot. Channel placement was switched, Left, Center, Right.
Hardcore Beatles people will probably say something different but if I remember correctly, the "stereo" albums you describe were really never meant to be stereo. George Martin would record in a manner such that the entire rhythm section might be on one track, with a track for lead vocal, a track for backing vocals, etc. By combining some of these tracks in a "bounce," he then had more tracks. As I understand, George Martin intended the tracks to be summed and released on a monaural disc. At some point someone realized the tracks were separated enough that they could be two channel mono and called "stereo."
If you want a great education, listen to either the left or right track of any song. It's a revelation. In fact, the original SGT. Pepper was mixed and released in mono. To my mind, that remains the definitive mix and they one all of us (of a certain age) remember when the chicken turned into a guitar.
Barry