Countermark

https://rpc.ashmus.ox.ac.uk/countermark/809 
GIC 688
Φ (on reverese)
shaped 
c. 7.5 mm 
Φ for Philus 

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber
RPC I, 1207a Show 1207 16 1
RPC I, 1207b Show 1207 3 1