Countermark

https://rpc.ashmus.ox.ac.uk/countermark/821 
GIC 702
D?
Sometimes on reverse 

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber
RPC I, 1629 Show 1629 16 1
RPC I, 5421 Show 5421 12 1
RPC I, 1555 Show 1555 64 1