A slightly different rotating notch rear sight plate is common to many higher end Diana's. Right through to the late 80's at least. Pull and turn with the fingers easily.
A very nice Model 50 you have. Superb find.
I have an old Original model 50, .22, with the same sight set up and has a "50M" type stock. It is marked as Original Model 50 with no prefix to denote what the exact model is. I can't see any serial numbers but I would guess it's a 1950's air rifle. Any one have any idea what these rifles are worth these days
I'll see if I can get some photos