Your rifle appears to be made by Samuel Mier (1831-1909), a late Somerset County, PA gunsmith. His father Jacob Mier and brother John Mier were also gunsmiths of some reknown. Samuel worked in Elk Lick Twp. in Somerset Co., and substantial information has been published on him, much of it by Jim Whisker in his "Gunsmiths of Bedford, Fulton, Huntingdon and Somerset Counties."
The gun, despite its damage and relatively late style, has several details that indicate it was made by Samuel Mier. These details include: 1) signature, with letters cut in Mier's style and shape, 2) cheekpiece with multiple molding lines and rounded lower edge, 3) engraved pattern on lid, including the serpentine lines along either edge of the lid, 4) oversized release button in toe plate, 5) non-standard hinge configuration, 6) use of non-standard side leaves on his patchboxes, which were highly engraved and each was very individualistic. The gun can solidly be attributed to Samuel Mier, and is a relatively late gun by him, probably dating to the 1860s or later. He also used large silver inlays in the cheekpiece, usually with an eagle engraved on it... but I can't see what figure is engraved on your cheekpiece inlay. Shelby Gallien