A nice piece.
Coin by the authors of RIC appreciated by the R4 rarity grade denoting 2-3 pieces known to them.
Obverse: august head in rosette diadem, to the right, in the rim legend:
CONSTAN-TINVS AVG;
Reverse: camp gate of 9 rows of bricks, with open gate, in the rim legend:
PROVIDEN-TIAE AVGG; in SMANTB section;
Bronze, diameter 19 mm, weight 3.99 g, axis 6 h.