https://rpc.ashmus.ox.ac.uk/countermark/908 

GIC 781

denominational mark Δ

circular 

7 mm 

Marcianopolis (Moesia)

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