nice find, here in the 1931 catalog

most to all pre war guns ( pre WW 2) were made for the 65 mm long shell so its possible and happen many times that an old gun was rechambered for the 70 mm long shell when it became the new "normal".
don't know enough about proof marks but thinking when this is "16 Normal" and the 65 mm long shell was "Normal"then it would get another mark in the proof house showing it was born as a gun for 70 mm long shells.

don't know what for a bullet it was made for then, here are some in the 1900 Gevelot catalog

I would start with a round ball and then try a 16 ga paradox bullet if you don't want to use modern slug bullets.