Probably an inspector's mark. I have a Belgian rifle, dated 1859, that's covered with inspector's marks and proof marks. Apparently, everytime someone inspected it, they stamped it, and then their boss stamped it and probably their boss, too. I counted about 20 on the lockwork itself. These are some of the marks on the barrel.
As far as your shotgun - is there anyone in your area who can look at it and appraise it or tell you how to restore it? Gotta love those old guns.