Countermark

https://rpc.ashmus.ox.ac.uk/countermark/600 
GIC 473
thunderbolt
rectangular 
7 x 4 mm 
Seleucia (Syria)
The thunderbolt is a standard symbol for Seleucia (a cult object of Zeus Keraunios), so probably a local cmk, rather than one of XII Fulminata 
List of countermarks View on a map

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber
RPC I, 4330 Show 4330 20 1