Countermark

https://rpc.ashmus.ox.ac.uk/countermark/790 
GIC 662
Α?
circular 
6 mm 
as diameters of coins go from 23 to 31 mm, and weights from 6.72 to 18.23g, unlikely to be a denominational mark 

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber
RPC V.3, — (number not assigned), type ID 74915 Show 720 1 5.3
RPC VI, 6070 (temporary) Show 5876 5 6
RPC VI, 6031 (temporary) Show 5827 4 6
RPC VI, 6059 (temporary) Show 5859 1 6
RPC VI, 6014 (temporary) Show 5812 1 6
RPC V.3, — (number not assigned), type ID 74905 Show 709 3 5.3
RPC VI, 6037 (temporary) Show 5846 7 6
RPC VI, 6051 (temporary) Show 5857 3 6