Countermark

https://rpc.ashmus.ox.ac.uk/countermark/761 
GIC 638
monogram ΤΟ ΚΡΑ
ΤΟ ΚΡΑ = year 121. On reverse 

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber
RPC VI, 5652 (temporary) Show 5127 2 6
RPC VI, 5501 (temporary) Show 4978 18 6