No offense Cosmoline but the 'fox-in-the-circle' (as well as the tombstone fox' are marks of the firearms made for the big trade companies for Native American sales. The Hudsons Bay Company is a prime example.
For anyone who wishes to combine business with pleasure (or vacation with pleasure), stop by the Museum of the Fur Trade in Chadron, Nebraska. They have the largest collection extant of the different versions of Trade Guns.
Back in the 60s and 70s I used to meet up with Curly at various rendezvous' and paw through his wares. He was the first one (other than custom) to put some effort into recreating the NW Trade Gun. It went by the name of Northstar. The company 'North Star West', which is a direct descendant of Curly's original organization is located at: http://www.northstarwest.com/
There are many books on the Hudsons Bay Company, North West Company and American Trade Companies that are very informative and include details on this subject and many others. Good winter reading.