Intriguing.
I have played the bass for All Day in Ab and it's less straightforward than it would be in A. Simplicity would have been more likely in those days, as would the key of A.
You'll remember from the Abbey Road offcuts CD that when the earlier version of All Day was recorded, Bruce said he was going to slow it down. We then hear it played in (roughly) A. Here is a comparison between that version and the later, rearranged, polished version in (roughly) Ab. Note that the speeds are similar
Here is the same comparison, but this time with the later version speeded up to (roughly ) A. Now the keys match but the tempo of the later version becomes faster.
My theory is that they polished up the arrangement and recorded it in A. It then appeared faster than they wanted so they ran the tape a little slower.
Only a theory
