This is the take that ended up being the master, but it's mostly Paul on piano and vocal and Ringo playing brushes.
According the Mark Lewisohn's The Complete Beatles Recording Sessions, a number of tracks were added at another session later that evening including Paul's bass, John and George on fuzz guitar (through the same amp no less), Ringo's drums and another vocal by Paul.
The song was finished a few days later with John and Paul backing vocals and a sax section.
1) Paul is a really good rock piano player. Even though a second piano was supposedly recorded later, you have to wonder exactly what was added since this track is very solid already. If anything, I suspect that the part between the bridges might've been fixed, since he plays it a bit differently from the main part.
2) Notice the handclaps in the background at about 1;37. According to Lewisohn, there were more added at a later date.
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