I bumped into this footnote in Ian McDonalds Revolution In The Head (which does contain notable erros by the way. For example: it says that I'll Follow The Sun was written in Hamburg. But it was recorded on a tape machine by The Quarrymen before they went to Germany). The footnote I mentioned is for this song. I quote, complete with the formentioned error: "Written during the summer of 1960 in Hamburg, it's a regretful if ultimately rather callous ditty composed like When I'm 64, as a diversion from nights of noisy rock-and-roll. According to former Beatles drummer Pete Best, the 18-year-old McCartney would play it on a piano between sets at the Kaiserkeller, announcing cheerily to anyone in earshot 'I've written a song'. Footnot: 'At this juncture, McCartney had temporarily become The Beatles' pianist, his cheap Rosetti Solid 7 guitar having rapidly succumbed to the rigours of knockabout performances in sweaty fleapits and fallen to pieces. When, within weeks, Lennon bought his Rickenbacker 325, he gave McCartney the single pickup Hofner Club 40 he'd acquired in Hamburg a few months earlier.'
Not dated correctly in my opinion, but the pickup idea might be interersting.