I haven’t spent a lot of time (actually, none) worrying about the level of proof on my French guns. The barrel diameter, measured 9” from the breech, was stamped on the flats. Measure the bores to see if they have been tampered with, if not, use the gun with the appropriate ammunition. The proof level is higher and the wall thickness typically is as well, to pass that higher proof.

Typical hunting loads in my world aren’t going to hurt a French gun that remains in proof.

Best,
Ted