Countermark

https://rpc.ashmus.ox.ac.uk/countermark/789 
GIC 661
Α
rectangular 
4 x 3.5 mm 
compare GIC 716. GIC had A? but 2nd example seems to confirm A 

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber
RPC III, 2811 Show 2811 2 3
RPC III, 2811 Show 2811 1 3