Countermark

https://rpc.ashmus.ox.ac.uk/countermark/1364 
GIC
Α or Δ
circular 
all GIC 747 and 788 cmk's seem to have been applied to the obverse. This is created as a) there are no other cmk's noted for Cyzicus and b) it is on the reverse 

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber