Just a little story I was told the other day...
... about a fan who in the sixties had got Paul to sign a photo of the Beatles, then managed at a later show to get Ringo to sign alongside.
She missed out on John, but caught up with George years later, at an event in the eighties.
As George was signing alongside Paul and Ringo, she commented that she was hoping to get all four, but of course that was not to be....
Quick as a flash, George said "Oh, I can do John for you if you like"
The four of the Beatles had to sign so many photos etc in the 60s, they all learned how to do each others' signatures.
(If you look on UK ebay at the moment, you'll see a sixties Beatles photo which Paul has signed as all four of them)