Scott they are primarily found on rifles of the Northampton Co. area, however also there is some bleed-over into eastern Berks Co. although most (if not all) of what is found in eastern Berks is much more 'folky' or primitive. Also the only times I've seen such a head with what is clearly a feather - as opposed to a cap - have been on rifles of eastern Berks.
I have long felt that the origins of this symbol being used upon Northampton rifles are somehow tied to the fact that a fairly substantial, busy armory was established in Allentown ca. 1777-1779, and the use of this symbol was in some way a commemoration of Revolutionary spirit. Thus far, the earliest I have seen are upon arms signed by or otherwise attributed to the Moll family. John Sr. was long-established in Allentown by the time the War broke out, and was present during the time the armory was in operation although whether he worked directly for Cowell/Tyler or aided the effort in his own shop - or some combination of the two - is still a mystery. I do believe he was the progenitor of this symbol being used on arms, and because I also believe he trained at least some of the others that utilized it, I think it somehow was being used after the War as something of a commemorative "brand."