Nero AE As Overstrike on Galba by uncertain Balkan mint" 

NERO CLAVDIVS CAESAR AVG PM TR P - Laureate head right with overstrike GALBA in Greek
S C - Providentia standing left, holding globe and scepter.

Uncertain Balkan mint: AD 54-68 = RIC I, 

The countermark GALBA in Greek may have been used in the 
frontier area occupied by the legions of Upper and Lower Moesia, 
which lay far away from existing Aes mints.
RIC I, page 187 , 8.34 g.

Ancient Auction House