I’d say its a well made bag of some sort.
It looks like with the right color dye, along with properly stitched leather strap it would lend itself well as a fowlers bag that any upland hunter would be proud to own, or a rifle man sportsman . Then again I am partial to well made and properly stitched leather goods. As I believe not every mountain man, Frontiersman just threw couple pieces of leather together to come up with a possible bag . Of course there were plenty that did, but at the same time I also believe they took pride in their workmanship equal to the gun they carried..JMO.
Twice.