I have a 1st Model Single .22 shot #23694 with matching numbers on the front grip strap, barrel, latch, and in pencil on the inside of the right hard-rubber extension stock. The 10" barrel is marked "Model of 91" and the serial number is followed by a "B". Neal & Jinks lists .22 barrel serial numbers for this model, but none in this serial number range - the highest is #18873. What am I not understanding, and what is the significance of the "B" after the serial number on the barrel?