Good thoughts on the Dunmyer - Knupp connection on this rifle. J. Dunmeyer died in 1885 when C. Knupp was only 22 years old. So, even though they were master - apprentice and uncle - nephew, their working years only overlapped by a very few years. I did notice that the lock plate engraving, that is the decoration/shading, was clearly by the same hand as the patchbox engraving. However, the initials seem to be engraved by a different gunsmith. Or, at least, this is a possibility. I tried my best to see any marking on the top barrel flat, but there is none there, too bad.
Indeed, as you suggest, this may be an unsigned J. Dunmeyer that was owned by his nephew - apprentice, C. M. Knupp. Too bad the gun can't talk!
As to Dan's remark about the museum - collector issue, happily the folks in Somerset are clearly knowledgeable about the guns in their collection, perhaps unlike big city museums. Most folks out here in the woods know how to care for their guns, and also how to get meat with them.
Jim