Countermark

https://rpc.ashmus.ox.ac.uk/countermark/770 
GIC 647
monogram CAESAR or CAE or CAR
Caesar. Sometimes on reverse. 

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber
RPC I, 849 Show 849 14 1
RPC I, 850 Show 850 2 1
RPC I, 851 Show 851 5 1
RPC I, 851 Show 851 8 1
RPC I, 851 Show 851 10 1
RPC I, 851 Show 851 9 1