What a beautiful song! It's amazing to hear the demo (no drums) and after that the final version (with drums added). It already sounded very good in the demo but more of the song's vitality and nuance was brought out as they completed the song. I like how the tempo changed near the middle and how, I think Paul(?) reacted with an excited falsetto sounding and approving "yes." John was a magnificent songwriter, and his voice was perfect for whatever he sang. He was also so great at extemporaneous dialogue (at the end). So creative (consider Give Peace a Chance by comparison). His words flowed so naturally forth. What an amazing quick-witted intelligence he had.
The drums really added a lot to this song. The guitars closing is so tender (aww). It touches my soul.
The straight drumbeat could have been Paul, but the high-hat work sounded more skillful to me. But yes, the steady hard beat certainly could be Paul. So, I'm leaning for Paul and not for a Ringo overdub.
But the fills at the end (very Ringo) but also somewhat muted (Paul?). Closing the song with a solid steady beat and then none ...
My verdict - I think Paul. I think they didn't want the drums to be very prominent in the mix as they wanted the focus more on the pretty melodies and tender mood of the song.