Dave,
I'm not certain that a touch mark on a lock plate would indicate the maker of the rifle, usually not, but could be. I don't know where your grandfather lived but I have a listing for a John Briggs Hixon in Antrim, Guernsey County, Ohio from 1822-1899. His touch mark consists of an arrow with 2 vertical lines through it, forming the letter "H". Hope this helps a little.
Mark