Countermark

https://rpc.ashmus.ox.ac.uk/countermark/685 
GIC 556
ΠΡ(Χ?)
rectangular 
7.5 x 5 mm 
might have been applied at Prusa or Prusias; was possibly to give a civic validation to the coins of Nero in AD 68/9?  

Coins with this countermark

Number Image TypeNumber CoinNumber VolumeNumber
RPC I, 2018 Show 2018 7 1