Countermark

https://rpc.ashmus.ox.ac.uk/countermark/1421 
GIC
B (in circular punch)
circular 
6 mm 
as both examples are on coins of Panormus, suggested that applied there? 

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber
RPC I, 643 Show 643 11 1
RPC I, 642 Show 642 33 1