Gentlemen, Rich may be almost correct about the eastern area of Pa. assessment, however, the top portion of the casting suggests an earlier period than found on Bucks Co. or Lehigh Valley guns. Suggest you examine known early rifles from Reading and Lancaster that are a little further west to solve the problem.
Good examples can be found in the pre-Rev War sections of Shumway and Kindig. Pay specific attention to the facets on the top of the butt plate casting and the width of the butt plate.
Perfect for an 1760 style rifle. Regards, HIB