https://rpc.ashmus.ox.ac.uk/countermark/642 

GIC 515

ΑΜΙΣΟΥ

rectangular 

11.5 x 3.5 mm 

Amisus (Pontus)

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.