Countermark

https://rpc.ashmus.ox.ac.uk/countermark/756 
GIC 633
monogram ΡΚΑ?
Circular punch 

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber
RPC I, 1577 Show 1577 2 1
RPC I, 1567 Show 1567 36 1
RPC I, 1568 Show 1568 45 1
RPC I, 1578 Show 1578 31 1