Countermark

https://rpc.ashmus.ox.ac.uk/countermark/740 
GIC 615
monogram ΔΙΟΓ
square 
7 mm 
Hadrianotherae (Mysia)
Interpreted as being for the magistrate Diog(enes). This means that Diogenese countermarked his own coins, but there is nothing inherently wrong with that. 
List of countermarks View on a map

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber