All the old stuff in the UK till about 1968ish was mixed properly for mono - to sound good through a dodgy one speaker radio on the kitchen bench. Its why Sergeant Pepper sounds amazing in mono and kind of odd / lackluster in stereo. But by The White Album they did a proper job on stereo too. Most stereo mixes were just rushed and done with no vision that this would be the dominant listening experience of the future.
So early Stones in mono makes sense - I was once in a recording studio in Sydney (how important do i sound already?) and the engineer put on Sympathy For The Devil - when he unpressed the 'mono' button the piano is out of phase in each channel and cancels itself out - you cant hear it! Quite amazing.
