If actually the head of a screw, there would be few reasons for it. In the (much later) era of automated mold-making, a pattern suspected of producing defects would have a screw driven into it to confirm which pieces it produced. In (much) earlier times, one might expect to see something like that if a pattern was broken and repaired, but you would think they'd have been a little more concerned than to leave it that apparent, not to mention that being an unlikely way to repair a break in that particular spot. If neither of those, it could be an unintentional impression made in the surface of the sand mold before the iron was poured, which would have resulted in a raised area on the casting.