If the serial # was already in the importers system, they had to add a suffix or prefix to make a "new" serial number. Not all of the rifles received this treatmet. Nobody likes added stuff but it's really tiny and the alternative was dropping $3000+ for one of the earlier 1990s imports. I kept a 937B myself and it has the pb, doesn't bother me in the slightest. I'd rather have that than the typical huge ****ing billboard PW and century used to put on the side of the receiver.
With the 937A models, an A suffix was added.