https://rpc.ashmus.ox.ac.uk/countermark/90 

GIC 818

denominational mark Ϛ

oval 

7 x 5 mm 

Perge (Pamphylia)

Probably a distinct group. It might belong in GIC 815, but the style of the numeral is different and Perge lies well outside the area from which the coins of 815 were drawn