Countermark

https://rpc.ashmus.ox.ac.uk/countermark/1612 
GIC
IMP monogram (in rectangular or square punch)
rectangular (or square?) 
5.5 x 5 mm 
different from GIC 649 as on a later coin and the shape of the punch is different 

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber
RPC II, 660b Show 660 4 2