Welcome! First, that is not a musket, but a double barreled muzzleloading shotgun. The term acier (or d'acier) is French for steel. Do not think that is the MAKE of your gun, but the composition of the barrels (Twist steel) The barrels appear to be Damascus, or "twist" steel- which was pretty good for black powder- when they were made- about 150 yrs ago. From the rust showing in the pictures, I would not shoot this with ANYTHING- black or nitro. Damascus barrels did not age well, especially when exposed to corrosive effects of black powder. IF you can remove the forearm, you may find some markings there that would give a clue- makers name, or proof marks. You might also run a google search for images of French shotguns, such as the Darne, and see if anything matches up. BTW, if you can find a product called KROIL, it is the best penetrating oil I have ever seen- can actually get under rust and loosen it without scrubbing that will kill collector value. Not cheap, works incredible well.