Thessalonica didn't issue any coins with "M' in the left field with the the legend on the OP's coin.
DN IVLIANVS NOB CAES.
Style aside, according to the spread sheets Siscia is the only mint that issued coins with the unbroken DN IVLIANVS NOB CAES legend AND with the "M" in the left field.
Maybe a babrbaric type?