Countermark

https://rpc.ashmus.ox.ac.uk/countermark/949 
GIC 835
denominational mark Ι
incuse in shape of I 
7 mm tall x 1.5 mm wide (at widest point) 
The cmks were probably contemporary with the coins of Valerian and Gallienus which had an I on the die, as the Γ of GIC 778. 

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber
RPC VI, 5999 (temporary) Show 5641 3 6