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 5712 3 6
RPC VI, 5992 (temporary) Show 5708 1 6
RPC V.3, — (number not assigned), type ID 73766 Show 371 14 5.3
RPC V.3, — (number not assigned), type ID 73774 Show 373 4 5.3
RPC V.3, — (number not assigned), type ID 73776 Show 374 2 5.3
RPC V.3, — (number not assigned), type ID 73767 Show 369 6 5.3
RPC V.3, — (number not assigned), type ID 73767 Show 369 8 5.3
RPC V.3, — (number not assigned), type ID 73768 Show 370 2 5.3
RPC V.3, — (number not assigned), type ID 73768 Show 370 1 5.3
RPC V.3, — (number not assigned), type ID 73775 Show 372 1 5.3
RPC V.3, — (number not assigned), type ID 73774 Show 373 3 5.3
RPC V.3, — (number not assigned), type ID 73773 Show 375 2 5.3
RPC V.3, — (number not assigned), type ID 73767 Show 369 1 5.3
RPC V.3, — (number not assigned), type ID 73774 Show 373 2 5.3
RPC V.3, — (number not assigned), type ID 73774 Show 373 1 5.3
RPC V.3, — (number not assigned), type ID 73766 Show 371 6 5.3
RPC VI, 10982 (temporary) Show 5709 1 6
RPC V.3, — (number not assigned), type ID 73767 Show 369 5 5.3
RPC V.3, — (number not assigned), type ID 73768 Show 370 3 5.3
RPC VI, 5995 (temporary) Show 5712 2 6
RPC V.3, — (number not assigned), type ID 73766 Show 371 16 5.3