Countermark

https://rpc.ashmus.ox.ac.uk/countermark/906 
GIC 778
denominational mark Γ
incuse 
6 mm (vertical) x 4 mm (lateral) 
The cmks were probably contemporary with the coins of Gallienus and Salonina which had a Γ in the reverse field, as the I of GIC 835 

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber
RPC VI, 5995 (temporary) Show 5676 3 6
RPC VI, 5992 (temporary) Show 5672 1 6
RPC V.3, — (number not assigned), type ID 73766 Show 351 14 5.3
RPC V.3, — (number not assigned), type ID 73774 Show 353 4 5.3
RPC V.3, — (number not assigned), type ID 73776 Show 354 2 5.3
RPC V.3, — (number not assigned), type ID 73767 Show 349 6 5.3
RPC V.3, — (number not assigned), type ID 73767 Show 349 8 5.3
RPC V.3, — (number not assigned), type ID 73768 Show 350 2 5.3
RPC V.3, — (number not assigned), type ID 73768 Show 350 1 5.3
RPC V.3, — (number not assigned), type ID 73775 Show 352 1 5.3
RPC V.3, — (number not assigned), type ID 73774 Show 353 3 5.3
RPC V.3, — (number not assigned), type ID 73773 Show 355 2 5.3
RPC V.3, — (number not assigned), type ID 73767 Show 349 1 5.3
RPC V.3, — (number not assigned), type ID 73774 Show 353 2 5.3
RPC V.3, — (number not assigned), type ID 73774 Show 353 1 5.3
RPC V.3, — (number not assigned), type ID 73766 Show 351 6 5.3
RPC VI, 10982 (temporary) Show 5673 1 6
RPC V.3, — (number not assigned), type ID 73767 Show 349 5 5.3
RPC V.3, — (number not assigned), type ID 73768 Show 350 3 5.3
RPC VI, 5995 (temporary) Show 5676 2 6
RPC V.3, — (number not assigned), type ID 73766 Show 351 16 5.3