Countermark

https://rpc.ashmus.ox.ac.uk/countermark/761 
GIC 638
monogram ΤΟ ΚΡΑ (on reverse)
circular 
6–6.5 mm 
ΤΟ ΚΡΑ = year 121. GIC gave BMC 239 as having GIC 638, Reassigned to GIC 631 

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber
RPC VI, 5652 (temporary) Show 5190 2 6
RPC VI, 5501 (temporary) Show 5034 19 6
RPC VI, 5501 (temporary) Show 5034 18 6