I am far from an expert, keep that in mind. So if i understand correctly, you have an FN mauser in 7.92. While i do not know what the 6 pointed star and B mean. (unit mark or some such.) I do know that weapons that passed through Britishicon hands (military and civilian.) were proofed in Britain and marked with the corresponding pressure per square inch on it. I havent seen an FN made weapon marked with pressure testing originating from Belgium. Plus FN weapons are usually marked in Frenchicon.
A while back on GB i saw a ww1 captured 1898 that had the tons per square inch on it that was proofed in Britain and then used by Canadians as training rifles.
hope that helps.