Countermark

https://rpc.ashmus.ox.ac.uk/countermark/599 
GIC 472
thunderbolt
Sometimes on reverse 

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber
RPC I, 4277 Show 4277 6 1
RPC I, 4280 Show 4280 9 1
RPC I, 4277 Show 4277 10 1