While FN did use the "A" prefix for initial post occupation built BHP pistols...built from German (Allemand) manufactured occupation parts, FN did not use a simple "B" prefix SN at all.
On the other hand, the Hungarian firm FEG...now defunct, built copies of FNs BHP for years using not only the FEG slide legend, but other slide legends such as "Luger M80", "Parabellum", "Mauser-Werke M80SA" and the US importer Kassnar. With the exception of the "Mauser-Werke" copies that used an 8000 prefix SN, the SN of the other copies are to be found with either a "B" or "L" prefix...followed by 5 numbers. BTW, these FEG P9/M80/PJK-9HP BHP copies are good, well made, pistols that accept FN BHP parts.
In the early/mid '90s FEG covertly built outright counterfeit copies of the FN BHP ('73 Pattern) for an "End-User" country that was no longer able to get legitimate BHP pistols from FN. FEG went so far as to copy the FN slide legend and even obvious "proofs" including the FN "pineapple" on the trigger guard. BUT, apparently they didn't care that the placement, font and SN range they used was identical to that which they used for the FEG branded pistols mentioned above. Because of these obvious...and other, markers FEGs counterfeits are easy to identify...if one knows what to look for.
BTW, the frame SN will be found either on the right side just below the ejection port or on the front strap of the grip. The SN was also stamped on the slide...below the ejection port and the barrel...visible through the ejection port. I mention that because many FEG pistols are to be found in the marked lately, some are parts guns built from a variety of FN, FEG and Inglis parts. If there is no matching...in font and number, SN on the slide and barrel, it'll be a parts gun. If the 3 SNs are identical in font and number it is one of the outright counterfeits.
B31114
IS such an FEG BHP copy or counterfeit, either in whole or as the frame of a parts gun. B31114 would have been manufactured in late '93/early "94.
Might not be what one might want to hear, but the information is correct.
Hope that "helps"