This coin looks like RIC VIII Sirmium 34, except for the size. Sirmium 34 is around 21- 22mm & 4.25gm; while this one is a reduced module of almost half the weight. Perhaps it was struck with the next issue, which are all fallen horseman types ( starting with Sirmium 48) and are circa 17- 19mm & 2.45gm.
Constantius Gallus
A.D. 351- 354
19x20mm 2.3gm
DN CONSTANTIVS IVN NOB C; bare- headed, draped and cuirassed bust right, delta behind bust.
CONCORDIA MILITVM, emperor diademed and in military dress, standing facing, head left, star above, holding labarum in each hand inscribed with a Chi-Rho.
in ex. SIRM
RIC VIII Sirmium --