Countermark

https://rpc.ashmus.ox.ac.uk/countermark/1620 
GIC
+ or X in circular punch, points of X touching the circumference (on obverse)
circular 
2.5 mm 
differs from GIC 691 as on earlier coins and circular, not square, punch 

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber
RPC I, 2856 Show 2856 23 1
RPC I, 2856 Show 2856 28 1