Countermark

https://rpc.ashmus.ox.ac.uk/countermark/603 
GIC 476
trident
rectangular 
6 mm wide 8 mm tall 
Corinth (Corinthia)
The cmk was not an act of Damnatio (a coin of Claudius was countermarked too) but equated older coins with the issues of the interregnum of AD 68, on the obverse of which appeared a head of Neptune with a trident. 
List of countermarks View on a map

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber
RPC I, 1770 Show 1770 8 1
RPC I, 1189 Show 1189 22 1
RPC I, 1205 Show 1205 34 1
RPC I, 1203 Show 1203 1 1