Countermark

https://rpc.ashmus.ox.ac.uk/countermark/488 
GIC 368
amphora
circular 
5 mm 
Sometimes on reverse 

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber
RPC I, 2303 Show 2303 2 1
RPC I, 2302 Show 2302 2 1
RPC I, 2303 Show 2303 13 1