Watch List notes:

MACEDON. Uncertain mint. early to mid 5th century BC. Obol (Silver, 13 mm, 0.60 g). Crab. Rev. Irregular incuse square. HPM pl. XV, 16. Tzamalis 5. Rare. Very sharply struck and beautifully toned. Extremely fine.
From the "Collection sans Pareille" of Ancient Greek Fractions, ex Hauck & Aufhäuser 14, 6 October 1998, 34.

Often attributed to Kos, the fact that coins of this type are commonly found in Macedonia makes it much more likely that it comes from a mint in that area.