https://rpc.ashmus.ox.ac.uk/countermark/148 

GIC 28

[CA]N, emperor draped bust l.

circular (or slightly oval) 

10 mm 

Antioch (Pisidia)

The shared cmk with GIC 18 gives the Pisidian Antioch origin. The inscription may read C(ol.) AN(tioch), although the poor condition does not allow certainty