Augie the first early rifle`s didnt have the screw to lock the magazine I have a c series argentine 1891 longrifle that is the same way, Im not sure when they added it but my other 2 a k and p series have it, you may also notice that the extractor location is different too if the bolt is original to the rifle it may be dead center between the lugs ,on the later rifles it was moved closed to the bottom lug, some other changes on later rifles are the handguard length, bolt sleeve with wings, and reinforced cocking pieces. Aaron10