Constantine II, AE3, 337-348 AD, Constantinople.

DN CONSTAN-TINVS PF AVG, pearl and rosette-diademed head
only, right.
GLOR-IA EXERC-ITVS dot, two soldiers holding spears and 
shields with one standard between them. O on banner.

Mintmark: CONSS. 

RIC VIII Constantinople 24 var (unlisted officina), R4.

From Dane's collection, August 2010.

Known mintmarks for this RIC number:
CONSB
CONS gamma
CONS delta
CONSS <- this coin, unlisted in RIC
CONSI
CONSIA