Update: I got the coin. What looks like the graffiti of a B in the picture is exactly what I suspected, two little spots from probably some old mineral deposit while it was buried or some other natural process, and not something more unsightly and man-made. I should add that although the coin is essentially XF+ or AU, there is a fair bit of crystalization of the silver (although not extreme by any measure), which is not unusual in these old coins. Even my Kyzicos boar hemiobol has crystalization. So yeah, beautiful coin, but best handled with care and not dropped.