https://rpc.ashmus.ox.ac.uk/countermark/131 

GIC 13

laureate head of Heracles, bearded, r.

oval or circular 

8 x 6.5 mm 

Latin coinage of Thrace and Bithynia (Uncertain)

GIC had Heracles(?) - subsequent example show it is Heracles. Martini-Pangerl 99, noted as applied to coinage in Moesia & Thrace