There are details on this rifle that look a lot like the work of Samuel Smith, a gunsmith who worked in Astoria, Fulton County, Illinois, from about 1850 -1872. The incised line from the toe to the trigger guard, as well as the rear sight and front sight, all look familiar. The brass base on the front sight, with four punch marks, is identical to those on Samuel Smith rifles. The architecture is very much like Smith's. The trigger guard isn't quite right, but as you said, it is a replacement. He usually signed his rifles with a script "S S", but not always. I have one unsigned one and three that are signed.