Countermark

https://rpc.ashmus.ox.ac.uk/countermark/611 
GIC 486
?, 'shield', possibly a 'Θ'
Sometimes on reverse 

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber
RPC I, 1626 Show 1626 14 1
RPC I, 1626 Show 1626 16 1
RPC I, 1541 Show 1541 7 1
RPC I, 1626 Show 1626 27 1
RPC I, 1629 Show 1629 24 1
RPC I, 1626 Show 1626 22 1
RPC I, 1626 Show 1626 30 1