A question for Henri Pieper owners.

The barrel flats of my Pieper (Bayard) hammer gun has a number of stamps; the usual proof marks including two 4-digit numbers - 9731 and 5491. It also has what looks like the Roman numeral XII.

The 9731 looks like a proof mark date. What does the 5491 (pressure?) and XII stamps refer to?

Thanks for any information.

Vincent