Countermark

https://rpc.ashmus.ox.ac.uk/countermark/934 
GIC 817 i
denominational mark Ϛ
oval 
6 x 5 mm 
all on coins from Lydia except the coin of Domna from Pisidian Antioch. Taken to be that 817i is a Lydian one 

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber
RPC VIII, — (number not assigned), type ID 20379 Show 1242 1 8
RPC VI, 5570 (temporary) Show 5099 6 6
RPC VIII, — (number not assigned), type ID 20661 Show 1750 12 8