Countermark

https://rpc.ashmus.ox.ac.uk/countermark/123 
GIC 7
Helios, radiate head r.
oval 
7 x 5 mm 
The countermark clearly belonged to Pergamum - 4 examples were found there. The Oxford coin has a facing head of Helios in the countermark. 

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber
RPC III, 1717 Show 1717 16 3
RPC III, 1728 Show 1728 1 3
RPC III, 1727 Show 1727 7 3
RPC III, 1726 Show 1726 3 3
RPC III, 1727 Show 1727 2 3
RPC III, 1727 Show 1727 4 3
RPC III, 1726 Show 1726 4 3
RPC III, 1727 Show 1727 5 3
RPC III, 1728 Show 1728 5 3