Countermark

https://rpc.ashmus.ox.ac.uk/countermark/736 
GIC 610
monogram ΒΤ
BT.. or ANTO..? 

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber
RPC IV, 4738 (temporary) Show 3901 3 4.1
RPC IV, 4738 (temporary) Show 3901 1 4.1