The Reynolds 531 seat stays that Peugeot used have a more rapid taper than their own stays, so they're larger in diameter at the big end. A dime centered on a 531 seat stay cap will protrude over the edges by less than a millimeter. It will hang way over on a non-531 stay. Take a dime with you when you go to look at a PX10, especially if it has been repainted. You'll avoid paying a PX10 price for a PR10. PR10's are nice bikes, but they're worth far less than PX10s.

The forks are easy. If it has a visible seam down the back, it's not Reynolds.