I’m a big music fan but I have to confess that I’ve never really understood the difference.
Mono: vox or guitar/drums/bass come out of one side but not the other side?
Stereo: everything comes out of the speakers equally on both sides?
I was reading about a bootleg of ‘The Who Sell Out’ in mono that says some of the songs have different guitar parts? Would they actually record different versions for each release?
What would be the difference between the mono and stereo version of say…‘Rubber Soul’ or ‘Pet Sounds’?
You’ve got that exactly reversed. A monophonic recording has only a single channel, and to feed a stereo amplifier, that channel is usually split to both the left and right inputs (otherwise, you’ll only hear the sound out of one side). A stereo recording has two separate channels which may or may not contain the same information. Differences in volume level or timing between one side and the other, and/or different instrument mixes produces the illusion of dimension, more or less mimicking what your ears would hear at a live show.
Mono is where everything is mixed to one channel, and comes out both speakers identically. Stereo is a different part of the mix in either channel. You had your definition backwards.
In the 1960s, most people had a mono record player. When stereo took hold, and the record companies started putting out stereo rock albums, the stereo editions were more expensive. They all made both mono and stereo mixes of most albums until around 1969. Sometimes they would just combine the stereo mix to one channel. Others, they would go back to the multitrack tapes and make a new stereo mix from the master tracks. This introduces the margin of error that makes for differences.
Because most people only had mono, and radio was mono, the groups and producers would spend a lot of time on the mono mixes, getting them absolutely right. More parts would be recorded and superimposed over the mono mix-in-progress, until they were all satisfied. Then later when they needed to put out a stereo mix, often these were done in a hurry, with little attention being paid to changes and edits to the original tracks that were done for the mono mix. Therefore, there are differences, sometimes subtle, sometimes massive, between the mono and stereo versions of albums.
The variations on Beatles records would fill an encyclopedia - and they have all been documented. It’s the same for a tremendous amount of music of the '60s. Collecting the different mixes has been my hobby for >35 years.