To guess the meaning of the 'B', It might help if the meaning of 'A' was understood. I don't spend as much time as others examining the variations of the Repro, but did notice that both the A, and B, stamped in the same location, are found on A1-Specials. Since the 'B' is rare, perhaps there's something in common with the origin of the barrels. Any of the many stamped with the A, are obviously of Japanese origin. My guess, as I said previously, is the barrels in the OP's opening post, are not of Japanese origin.