My belief is that this diminutive coin (a mere 12 mm across) is an issue of Valentinian II, and while the mint mark is rather indistinct, my best guess is that it's cataloged as RIC IX 65a (Thessalonica). Note the staurogram (tau-rho) in the lower right field on the reverse. I see these consistently misidentified as as a chi-rho (or christogram).

