According to this:
They used different methods to attach the rear sight base. Originally they used the screw to locate the base and silver-soldered it in place. One arsenal changed from the screw and solder to the two pins just before WWII, while the other arsenal kept to the old method. Later, some rifles had the two pins added, even though it had the screw and solder. So there are M91/30s with either or both methods of attaching the rear sight base.
Just out of curiosity, why are you removing it?