Tom, I think I might have read that somewhere, that Kuntz signed his name in different spellings, and that someone added the "T". But now that I have really looked at it, I agree with you. The finishing line of the "n" flows into the "t" - it would not flow into the "z" if the "t" were not there. So the "t" is not a later add-on. He did cramp his last two letters though...sucks to be mortal, don't it!!
Regarding the rifle, I think that the carving style, design and execution all point to the hand of Jacob Kuntz. I think that John Rupp was very tight with him, and that John had engraving and perhaps carving done on his rifles, by Jacob. This may be one such rifle. On the other hand, John Rupp may have been so impressed by Jacob's work, that he carefully did his in the same fashion. Who could ever stoop so low! I, for one.
It's all interesting,eh?