Countermark

https://rpc.ashmus.ox.ac.uk/countermark/909 
GIC 782
denominational mark Δ
Incuse. Sometimes on reverse; noted for coins of Nicopolis ad Istrum and Hadrianopolis 

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber
RPC IV, 4338 (temporary) Show 2559 7 4.1
Show 133
Show 200
Show 130
Show 193