According to conspiracy theories, this model it's been in use instead of sir Paul until today. And his name is Billy Shears...
there are 2 "different" things (although linked) :
1) the very well known "Paul is dead" hoax, with the Billy Shears substitution, all the "clues" on sleeves and in songs lyrics, etc...
2) the REAL fact that on the picture below, it's indeed NOT Paul who is back, because he wasn't present for this photo session, so he was replaced by a model. And among many other facts, this one served and maybe helped to start the famous hoax