I found this yesterday near Trenton, MO. The only thing I can imagine it could be is a leg band but I never knew of them having a rivet in them. Most or all are just wrapped around the leg and rely on the metal not bending during the birds lifetime. But this one having the rivet makes me think it may be something else. It has some info on it, "MO US APPR 1950 C15008". Its made of aluminum. Any ideas?