Countermark

https://rpc.ashmus.ox.ac.uk/countermark/1620 
GIC
+ or X in circular punch, points of X touching the circumference
circular 
2.5 mm and 5 mm 
Uncertain if the two diameters of punch are the same type? Applied on obverse; 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
RPC I, 2856 Show 2856 29 1
Show 438