For what it's worth, I have two Illinois-made rifles with that same trigger guard, both in brass, one on a rifle by Joseph Legler, Lockport, Illinois, the other on a rifle by L. Manker, Pittsfield, Illinois. I have four other Illinois-made rifles with very similar trigger guards. Two of those are by Henry P. Brunker, of Ottawa, Illinois. Brunker (as well as his local competitor, D. S. Ebersol) was also a brass founder. Both of them made their own mountings, and quite likely supplied castings to other gunsmiths in that area. I have seen a number of non-typical trigger guards, as well as butt plates, on guns by both of these two Ottawa makers.
Take a look at the two L. G. Ward, Springfield, Illinois, rifles that are pictured in the Library here. Both have extensive silver inlays, all engraved. One is signed "Fairthorne, Engraver", and is dated 1854. That one even has a similar trigger guard.