We'll need photos but Stoeger imported Belgian made trade muskets from at least the 1950s on. I became aware of them while I was in grade school and was given a Stoeger catalog ("The Shooter's Bible") showing them. There were several variations and they were originally intended for the African trade. Over the years other importers sold them too.
The longer barrel ones are very popular as wall hangers. The liquor store one town away from me has one. 2 piece flintlock made from a back action percussion lock, 60 inch long barrel! Owner turns down at least 8 people a year wanting to buy it.
My 1956 Stoegercatalog lists it as "Model 6494 elephant flintlock gun ....still used extensively today in the Belgian Congo. They call it a 4 gauge with 34 inch barrel and weighs about 9 lbs 14 ops. Selling price then was $36.
The one in the photo here. I remember Dixie selling these in their catalog back in the late 1960's and early 1970's. They called it an elephant gun. LOL The barrels varied between 4 and 6 gauge. The one in the photo is one of the better ones with the one-piece lock (versus two-piece). You will likely find the brass mounts are actually cast steel and brass plated.
rickystl has a good memory! Dixie sold these in their 1968 catalog as "Elephant gun". The sold them for $67.50 ( or the proofed Belgian barrel alone for $27.50). They also sold the mold for casting the ball and recommend about 200 to 225 grains of black powder. The earlier Stoeger ads stated that their guns were assembled with vintage locks and new made stocks, with gun to gun variability in the details
I bought one of those in 1963, as I remember the bore was rough as a cobb, loaded it up with some of Dupont's finest, newspaper wadding, tied it to a tire with a string from the trigger and got far far away. Pulled string, frizzen sparked, gun fired downrange and didn't blow up the barrel. I took it in the house and hung it on the wall. Mine was the "Zulu" model I think, very thin barrel almost like a shotgun. Don't think I would put an elephant load in it.
They also got to selling "kits", of which Dad bought one. If folks think some of these guns were crude (and some were), the kit was worse. True enough, if you took your time, had good hand tools, and Jupiter aligned with Mars - the end result would shoot for a while. The stock was a very light-colored, lightweight wood that came pre-warped and pre-cracked. Tiny back action lock, very large hammer, a few steel pins, steel ramrod with a nail head, 28 gauge percussion barrel. For many, those inexpensive guns were their first intro to black powder and provided a lot of fun.
As a 4 gauge, ( 0.90" bore ), it is probably restricted from game bird hunting. While it could be loaded down to an equivalent 10 or 12 gauge light load, I would be leery of using wads as the wad might blow through the shot column. Pattern checking is certainly advisable. It would be great for shooting blanks.