The presence of " Proof " marks means nothing. Stage props often use " stuff " bought in flea markets, or even overseas. And they have been known to reproduce those Proof marks. We have at least one very competent gun builder here, who has his own set of stamps to reprise the proof marks on replica guns he build, for instance. Its not done to commit fraud, but to replicate authenticity to the most minute detail.
Have someone knowledgeable about European made guns take a look at it, and be prepared with the necessary screwdrivers to remove the lock, and barrel from the stock, so he can look it all over. The lock was definitely made to be a percussion action, and that means that the flared muzzle is for a much earlier time period. This is the kind of thing that movie, and Stage prop houses do when they need a gun to fire a blank charge, but either don't know how to use a flintlock, or the Director is afraid that someone will be injuried and the gun won't fire if they use an actual flintlock action on the stage gun.
I have seen similar monstrosities used in opera performances. I can hardly contain my laughter, because I am the only one in the audience who understands how absurd the " Gun " is. Having experience doing off-stage sound and prop work for plays when I was younger( My brother and I were the only kids with any knowledge of guns, and other things, needed to make the right noise for school plays), The lack of knowledge, and the unwillingness by teachers, directors, producers, and actors to learn anything about guns is quite remarkable. I do find, in spite of all this, that many musicians are avid shooters, and they don't make that information known very often. I would learn about their " secret life " when they found out I am a gun person, and they would pull me aside, and quietly ask me where I shoot, and where they can get this or that.
Just don't expect to find any shooters in the Corps du ballet!