Watch List notes:

LESBOS. Methymna. Circa 500/480-460 BC. Hemiobol (Silver, 6.5 mm, 0.24 g, 1 h). ΜΑ - ΘΥ Facing head of Silenos. Rev. ΜΑ / Θ Rooster standing to right within a border of dots; all within a shallow incuse square. HGC 6, 1894. SNG Arıkantürk 675. Very rare. Sharply struck. Some roughness, otherwise, about extremely fine.
From the "Collection sans Pareille" of Ancient Greek Fractions.

Could the head on this coin be that of Acheloos?