Larry,
Kennet lists at least one mark for commercial re-proof of re-worked surplus military arms, circa 1862-1885.
I would be willing to bet that in the cases of French guns marked with one of the semi-smokeless powders (PJ, PS, PM, PR) that are also marked with the later PT, the later mark served as indication of reproof.
But, I don't know for sure. I have seen guns marked with proof for several kinds of powder.


Not many, however.

Best,
Ted