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 5762 3 6
RPC VI, 5992 (temporary) Show 5758 1 6
RPC V.3, — (number not assigned), type ID 73766 Show 390 15 5.3
RPC V.3, — (number not assigned), type ID 73774 Show 392 3 5.3
RPC V.3, — (number not assigned), type ID 73776 Show 393 2 5.3
RPC V.3, — (number not assigned), type ID 73767 Show 388 6 5.3
RPC V.3, — (number not assigned), type ID 73767 Show 388 8 5.3
RPC V.3, — (number not assigned), type ID 73768 Show 389 2 5.3
RPC V.3, — (number not assigned), type ID 73768 Show 389 1 5.3
RPC V.3, — (number not assigned), type ID 73775 Show 391 1 5.3
RPC V.3, — (number not assigned), type ID 73773 Show 394 3 5.3
RPC V.3, — (number not assigned), type ID 73767 Show 388 1 5.3
RPC V.3, — (number not assigned), type ID 73774 Show 392 2 5.3
RPC V.3, — (number not assigned), type ID 73774 Show 392 1 5.3
RPC V.3, — (number not assigned), type ID 73766 Show 390 7 5.3
RPC VI, 10982 (temporary) Show 5759 2 6
RPC V.3, — (number not assigned), type ID 73767 Show 388 5 5.3
RPC V.3, — (number not assigned), type ID 73768 Show 389 3 5.3
RPC VI, 5995 (temporary) Show 5762 2 6
RPC V.3, — (number not assigned), type ID 73766 Show 390 17 5.3