Countermark

https://rpc.ashmus.ox.ac.uk/countermark/908 
GIC 781
denominational mark Δ
applied to coins of Marcianopolis; A number of countermarks applied to the coins of Moesia Inferior and Thrace do not appear to belong to any of the groups from Asia Minor. They are listed individually, by city, in GIC 

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber