Watch List notes:

THRACE, Mesambria. Mid-late 5th century BC. AR Obol (7mm, 0.58 g). Corinthian helmet facing / Quadripartite incuse square. Karayotov I –; Topalov, Messambria –; HGC 3, 1562. VF, toned, slight granularity. Rare.

The absence of this issue from Karayotov and Topalov suggests that they did not consider it definitively placed at Mesambria, and does not reflect on its rarity.