incuse R (?)
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, omitted in error for RPC II, 901/2, but visible on the illustration at the top of the reverse. 

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber
RPC I, 2324A Show 2324 1 1