I can't see any sign of a filled hole for a frizzen pivot so I have to assume the lock started out as percussion. The way the drum fastens on makes me think the barrel may be a converted flint barrel. The hammer looks sort of north european rather than british. The french locks that I have seen have had maker's name or similar info engraved on rather than a deep stamping from a die, so that if there was any writing on the lock plate, it has probably been obliterated by rust now. Only other thing that I can think of is that if the barrel is of laminated metal, some french barrels were laminated up in layers and when browned looked sort of like plywood rather than the typical damascus twist that we are used to. Only way to find that out would be to remove the barrel and draw file the lower portion (ie covered by the stock) clean and try etching with copper sulphate or alternately brown it with a rusting solution
cheers mooncoon