[gdubs]

In some cases the mono mixes were the ones the Beatles themselves were present for. Some of the stereo mixes were the audio engineers experimenting on their own — this is why Beatles mono mixes are highly sought-after. Can’t recall whether Sgt. Pepper’s was a mono mix originally, but probably an easy search to find out — I’m too lazy to go to my bookshelf and flip through to the Beatles studio notes at the moment…

[PAGAN_WIZARD]

It was, and there are even some significant differences. On she's leaving home, the mono mix was speed up with the mono version's length coming in at 3:26 and the stereo at 3:35 [0]. I believe (running off of memory here too) that the first album where the stereo mix got some actual attention was the white album.

[0] https://en.wikipedia.org/wiki/She%27s_Leaving_Home

[websites2022]

Sgt. Pepper was indeed a mono mix originally, that was the version that the Beatles themselves oversaw. But, the new mixes are fantastic!