Countermark

https://rpc.ashmus.ox.ac.uk/countermark/145 
GIC 26 i
Eastern potentate, bust l., wearing diademed tiara, X (=M) behind
circular 
12 mm 
attribution to Edessa likely, but not certain 

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber
RPC I, 4246 Show 4246 9 1
Show 213
RPC III, 3482 Show 3482 12 3