https://rpc.ashmus.ox.ac.uk/countermark/1265 

GIC —

incuse R (?) (on obverse)

incuse 

6 x 2.5 mm 

Gargara (Troas)

A Latin letter would be a bit unexpected, so perhaps B is a possibility. KM 39, 7 mentions a K on its side ‘im Felde eingeritzt’ for a coin of Vespasian