In September of 352, Magnentius lost control of the Rome mint. The coin below was likely struck shortly after that date. The reverse is a type started by Magnentius and is fairly common for him and his brother Decentius; but less common for Constantius II and Gallus. It seems obvious that this type, continuing the old design of Magnentius, was the first type struck in Rome for Constantius II; but RIC says that there were some FEL TEMP coins possibly struck first, on the numismatic evidence of "Balkan" titulature, types and mintmarks "which all suggest Siscian derivation.". There is however, another possibility-- "one might conjecture that this coinage represents a brief revolt in Rome in favour of Constantius." (RIC VIII pg 243)
So the exact circumstances and time that this coin was minted are not known for sure; but I like the idea of a 'revolt' after Magnentius has left Rome.
Constantius II
A.D. 352- 354
20mm 3.0gm
DN CONSTAN-TIVS PF AVG; laurel and rosette diademed, draped and cuirassed.
VICT DD NN AVG ET CAES; two Victories holding wreath inscribed with VOT-XXX with Chi-Rho above.
In ex. R star T
RIC VIII Rome 252