Countermark

https://rpc.ashmus.ox.ac.uk/countermark/915 
GIC 794
denominational mark Δ
Circular punch, c. 7.5 mm. Sometimes on reverse 

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber
RPC VI, 4818 (temporary) Show 4027 3 6
RPC VI, 4401 (temporary) Show 3916 5 6