IIRC in my college days, if you played a track from Sgt. Pepper's Lovely Rita - towards the end of the song if you played just the left (or was it right?) channel - there were all moans and breathing - with no music. The other channel contained the music.

I know the mid 60's to late 60's Beatles experimented with different tracking and recording effects - but were the early albums only in mono - and then re-mixed to stereo?

Epic Grande Master 500 w/ on wall VP180