Ron James is correct; the so-called "bringback paper" was to satisfy U.S. customs, not to prove capture from the enemy.
There should be proof marks whether the gun was actually made in Belgium or France, though since fewer French guns are seen in the U.S., French proof marks are not as readily recognized.
The gun is a very typical large caliber European revolver of the era; the same basic design being made earlier in pinfire. There is nothing really unusual about it; the engraving is nice but not spectacular. It is probably in 10-11 mm, but there were several cartridges in that range, so further information would be needed to determine the exact caliber.
As to romantic ideas about gun used by the Resistance, I doubt any respectable Maquis would toss his STEN or No. 4 rifle in favor of an long-obsolete revolver for which no ammunition would be available.
Jim