Countermark

https://rpc.ashmus.ox.ac.uk/countermark/1539 
GIC
S in square punch
square 
3 x 3 mm 
see D W MacDowall “Countermarks of early imperial Corinth” NC⁷ (1962), 113-23 at 121 (pl.9, 1) 

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber
RPC I, 1119 Show 1119 50 1
RPC I, 1119 Show 1119 1 1
RPC I, 1119 Show 1119 3 1