Constantius II, AE3 of Rome. 

DN CONSTAN-TIVS PF AVG, laurel and rosette-diademed, draped, cuirassed bust right.
FEL TEMP RE-PARATIO, soldier spearing fallen horseman who is 
wearing a Phrygian helmet, clutching horse's neck.
S in left field.
Mintmark RP

RIC VIII Rome 267.
Rated rare.

Dane's collection, March 2016.