Constantius II. AE3, 25mm. 

Obv: DN CONSTAN-TIVS PF AVG, pearl-diademed, draped, cuirassed, holding globe; A behind bust.
Rev: FEL TEMP RE-PARATIO, soldier standing left, knee raised, spear
raised, over fallen horseman wearing Phrygian helmet, kneeling right on 
one knee, arm(s) raised.
Star over A in left field.
Mintmark R dot S dot B.

RIC VIII Rome 175 var.
Rare.

Dane's collection, Feb. 2011.

Note: this mintmark is listed only for RIC 255, BUT 255 has a gamma 
in the left field and no globe on the obverse. There is no doubt
as to the mintmark of this coin, so it isn't a 255.
RIC Rome 175 has this bust type and this reverse but has the mintmark 
R-dot-F-dot-B (listed) and R-dot-F-dot-Q (ebay sale, ca. 2008). 
So seeing as the bust type, field marks, reverse type, legend break,
correspond to 175 but only one letter (S instead of F) separates this 
from 175, this must be a 175 var.