I have two Colt 1917 revolvers. Both have serial numbers on the crane, but immediately below the serial numbers on both guns is a separate 4-digit number. Each revolver has a different number but on each gun the 4-digit number on the crane is also stamped on the barrel just below the "United States Property" mark. I'm guessing that they are armory issue numbers, but am not sure. Anybody know for sure?