You can reverse a one-piece base and hang the stop pin over the front edge of the rail. Old school trick for Diana's. That big, fat flat-headed screw on the rear of the later rails is for a peep sight and will shear off if used as a scope stop. You can also buy a scope stop. I just bought a Beeman Professional off of the Yellow yesterday. Very compact, flat and will lock down tight. Can't find a scope stop? Double the rear ring or cut down a ring and use the base for a stop. The Gamo stops will work but they are kind of cheesy, being largely plastic.