Countermark

https://rpc.ashmus.ox.ac.uk/countermark/1616 
GIC
A (rectangular punch)
rectangular 
6.5 x 5.5 mm 
see D W MacDowall “Countermarks of early imperial Corinth” NC⁷ (1962), 113-23 at 121 (pl.9, 3) 

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber
RPC I, 1118 Show 1118 1 1
RPC I, 1118 Show 1118 31 1
RPC I, 1118 Show 1118 3 1