I am having a bit of a hard time verifying the Rev Mint Mark. To me it appears to be SMNS, though I would like to verify with more experienced collectors.
It's a good guess. The mark is garbled either because of faulty engraving or die chips, or a weak strike. SMNE is also a possibility, as well as SMHS or SMHE. One of the maddening things about discerning certain Nicomedia from Heraclea mint marks is that the H in Heraclea mint marks often has a slanted cross bar, making the letter look like N, and the two mints used same format: Sacra Moneta, N or H, officina.
At this point your best bet may be to compare the styles of different mints during the 337-340 A.D. time span. (Constans is called Augustus rather than Caesar on your coin.) The reverse may be more useful than the obverse. I would start with Cyzicus, since a garbled SMKS seems most likely to me. Other mints to compare include Nicomedia, then Herclea, Antioch and finally, if all else fails, Thessalonica.
