I've got one of these too -- your
Herakles is nicer looking, though! I find it interesting that these
Akragas countermarks seemed to come in multiple different sizes (I think of yours as the large
type, mine as medium, and there are small ones). I wonder if there was any difference in meaning?
Mine is also heavily worn. The
obverse is probably totally blank (though a
bit encrusted). I'd bet some of that wear came after the countermark, but that the countermark protected the reverse/crab
side by deforming/cupping the
flan.
The host coin was struck ca. end of the 5th century BCE, I think. I wouldn't be surprised if the
Herakles was a couple hundred years later. I don't remember the references off the top of my
head, but I recall reading that
Republican AE
Asses (the 2nd cent BCE Janus/Prow ones) circulated for up to centuries, and then the really worn out ones could actually be exported en masse to regions with shortages of bronze coinage.
So I wouldn't be at all shocked if these were circulating all the way to the Roman periods in
Sicily (post-241 BCE). Hopefully someone will know, since I've wondered.
Unfortunately Greek countermarks are not nearly as well studied as
Roman Provincial ones, so there's no single reference like
Howgego is for
Roman Provincial countermarks.