Helena, AE follis, Thessalonica. AD 318-319. 

FL HELENA AVGVSTA, diademed, mantled bust right, wearing necklace.
SECVRITAS REIPVBLICE, Securitas standing left, lowering branch with 
right hand, raising hem of robe with left hand.

Mintmark SMTSB. 

RIC VII Thessalonica 159; Sear 16612.