Besides the information you have already on the proof marks, the barrel looks to me like it came from an Indian Trade Musket. Trade muskets were manufactured in a style that was demanded by the Indian customers, so most all of them that were built specific for the trade, even if made in different countries, all resembled one another, because the Indian customers demanded that the gun had a serpent side plate, a seated fox stamped into the lock, an extra large trigger guard etc., which to them indicated quality. However these gun were made quite cheaply, sometimes without a butt plate, and if they did have a butt plate, it was nailed on instead of using hand made screws, but the guns almost always had the brass serpent side plate. When reading about trade guns, you will see them called by a number of names, Hudson Bay guns, Hudson Bay Fukes, Northwest Guns are a few of the names, but they are all the same type of gun, and mean the same thing. These guns were the fore front of the frontier, and as civilization moved west, those Indians that were first encountered wanted this style of gun, and then later as they became more familiar with firearms, the trade rifle came into use. The plains Indians liked the smooth bore trade musket for hunting buffalo horseback, because they were easier to load on a galloping horse. Modern reproductions of the trade musket have the octagon to round barrel with wedding bands, but the barrels are straight, and the original muskets I've seen seem to all have tapered barrels, heavy at the breech and tapering to the muzzle. I googled Indian Trade Musket and there is some good information, but a lot of it is about modern reproductions. But if you search a bit, you should find some stuff that you might be interested in.