I am putting a scope on my 48. I undeerstand that a scope tends to creep on theses rifles. I noticed a screw head on the rear of the scope mount. Is this the stop or is this where you would mount a stop?

fits in one of the shallow holes on the mounting rail on the gun. The pin fitted in the hole acts as the stop. The screw you're referring to is, I believe, just holding the rail in place. Another alternative is one of the mounts (like some of the BKL's) that have 4 to 6 screws to tighten the mount to the rail. I originally had an RWS adjustable mount on my RWS48, but one of the adjustment lock screw holes stripped out, so I decided to try a BKL 260-D7 drooper mount, with 6 screws to tighten to the rail. It has no stop, but uses the tension of the screws to hold the mount in place. I've not had any problem with scope creep using the BKL mount.