On April 22 1966, a session at EMI Studio Number 2 saw The Beatles and George Martin clean up a couple of previously recorded songs via overdubs and edits.
The first tune worked on was "Taxman". This included erasing a couple of stray upwards bends on the guitar for the second verse (you can hear it on the previous "Taxman" post via the link I put up). Also, the "Anybody Got a Bit Of Money" vocal from John and Paul was taken out and replaced with the now famous "Mr. Wilson/ Mr. Heath" backing vocal in it's place. Cowbell was added to the song (more cowbell !!) and it's interesting to know that the mono mix of the song has the cowbell coming in about ten seconds earlier than on the stereo mix.
Lastly, the ending of the song was extended by taking the guitar solo from Paul and adding the entire sequence to the ending of the song in order for the tune to eventually fade out at the solo. The last syllable of the George lead vocal was brought in from a previous verse as well.
In addition, there was also work to be done on "Tomorrow Never Knows": the addition of an organ fading in and out and tambourine as well. The backwards guitar solo was added - again played by Paul - and John's lead vocal double tracked. The tack piano was also added at this session. You can hear this as the song fades out.
There are two mono mixes available commercially on the "Revolver" LP in the UK - the original mix was RM 11 which included artificial double tracking on the guitar solo and actually made it onto various early copies of the LP when it was released. George Martin decided to change the mono mix and RM 8 was used instead with no artificial double tracking on the solo. It has never been explained as to why the different mix was conducted and finalized when some of the LP vinyl was already pressed and ready for release. You can tell which copy of the LP you have by the run out groove number: the original mono mix has the code 606-1 on side 2 meaning it's remix 8, 606-2 means that you have the more common remix 11. Also interesting to note is that there is feedback heard right after the guitar solo on the stereo mixes, but eliminated during the mono mixes.