I don't know if they used it on the guitars, but according to Recording The Beatles, they used two EMI / REDD RS61 tube line amps in series to grunge up the horns on that song.
The session men were playing really well - there's nothing like a good brass section letting rip - and it sounded fantastic. But having got this really nice sound George turned to Ken Scott and said, 'Right, I want to distort it.' So I had to plug-up two high-gain amplifiers which overloaded and deliberately introduced a lot of distortion, completely tearing the sound to pieces and making it dirty.
The musicians came up to the control room to listen to a playback and George said to them, 'Before you listen I've got to apologise for what I've done to your beautiful sound. Please forgive me - but it's the way I want it!' I don't think they particularly enjoyed hearing their magnificent sound screwed up quite so much but they realised that this was what George wanted, and that it was their job to provide it.
Brian Gibson, engineer