I doubt literacy had much to do with whether a gunsmith signed, or did not sign, his guns, unless he lived in a very remote and poor area. Illiterate crafstmen usually had the skills to copy a signature someone else wrote down for them. If they could engrave scrolls, they surely could engrave a decent copy of a written name. A good case in point are the many signed Tansel powder horns by Tim Tansel, yet he was illiterate.
Many early Kentucky (state of) guns were unsigned. I think there were probably two primarly reasons. The first is, as Wayne mentioned, that in earlier years most guns were probably purchased and used locally, and the gunsmith's work basically "signed" itself.
I also think many early guns were looked upon by their owners as a basic working tool, or simple necessity, when homesteading on or near the frontier. Most such guns probably were not looked at as status symbols, as the higher art rifles made in more civilized areas were. Rather, many owners in the backwoods needed a basic rifle that stood up to the rigors of frontier life, just as they needed an axe, a plow, or a spinning wheel to survive. Many of those guns saw hard use, abuse, and often probaby eventualy destruction. It appears, in Kentucky at least, that until the frontier hardships had settled down and civilization really took hold, many guns went unsigned, particularly plainer ones which included a majority of the guns made there. There were exceptions, of course, like the fine Conrad and Michael Humble rifles and a few others. Shelby Gallien