This bass overdub replaced one which had been recorded two days previously. A reduction mix was then made, to free up space on the four-track tape.
The most famous of the overdubs was the backwards vocal part which adorned the coda, which featured Lennon singing "Rain... If the rain comes, they run and hide their heads." In one of his final interviews in 1980, Lennon claimed this as his idea.
I got home from the studio and I was stoned out of my mind on marijuana and, as I usually do, I listened to what I'd recorded that day. Somehow I got it on backwards and I sat there, transfixed, with the earphones on, with a big hash joint. I ran in the next day and said, 'I know what to do with it, I know... Listen to this!' So I made them all play it backwards.
All We Are Saying, David Sheff
However, in 1988 George Martin remembered it as having been his idea.
I was always playing around with tapes and I thought it might be fun to do something extra with John's voice. So I lifted a bit of his main vocal off the four-track, put it onto another spool, turned it around and then slid it back and forth until it fitted. John was out at the time but when he came back he was amazed. Again, it was backwards forever after that.
The Complete Beatles Recording Sessions, Mark Lewisohn
Listen to the released song's coda reversed, to hear the backwards vocals in their correct form:
With recording complete, four mono mixes of Rain were made, the third of which was selected for the single. During the mixes Lennon's lead vocals were thickened by artificial double tracking, or ADT.
There was no need for a stereo mix of Rain at this time. The song remained available only in mono until the February 1970 compilation Hey Jude, issued in the United States. The album featured new stereo mixes of Rain and Lady Madonna, created on 2 December 1969.