Countermark

https://rpc.ashmus.ox.ac.uk/countermark/642 
GIC 515
ΑΜΙΣΟΥ
countermark applied at Amisus. AMNG vol.1.2 p.615 n.2 - B Löbb. 1906; Oxford, 5.22 g; P 1245; L 1929,1013.495. Applied to very worn coins of the same size (18-21 mm). The countermark may have been Hellenistic or early imperial. The form of sigma Σ is last found on coins of Amisus under Trajan. On one coin (P.1245: https://gallica.bnf.fr/ark:/12148/btv1b85607256) there are traces of a bust of Tyche on the obverse. 

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber