I have nothing of merit to add, but I do own a few businesses and have never put a date on any of the signs. I have, however seen lots of signs added to a building with the build year as a historical marker. It would seen strange to me to have a date on a business sign. Also the lack of any business description seems suspect. You would think it would say "gunmaker", or "gunsmith" if it were a legitimate business shingle. My guess would be that sign was added somewhat later as a historical marker. That would not detract from how cool it is in my mind.
I appreciate this thoughtful response. It would have been very odd to put a date on a business sign. I suppose we see signs now that say "est. 1925" or whatever but that wasn't a practice during Rupp's time.
One does
sometimes see dates on stone German homes from this period, often near the top: I think they are called "datestones" (?), often have the initials of the owners, and they are placed among the structural stones high up on a house. (Not on business signs, that is, but set in the structure itself.) They're set high up on the structure, about where the photos from the 1960s show this sign being.
My current guess, though, is that somebody added this sign when it became a "depot" stop--for a stagecoach or a railroad--and included the "1817" to commemorate when the home was enlarged and the family member who enlarged it. So I think it was up there for a long while, though probably not since 1817.