I agree, Constantius II (discerned by the -IV- after the legend break). The reverse is FEL TEMP REPARATIO (happy times have come again) and it shows a roman soldier spearing a barbarian horseman (how else would they return the happy times?). The design is compacted here, but the flan is still relatively large, so its around the time when they reduced it (it was initially a follis sized thing, but quickly because smaller and smaller).