I would propose
SMTSB.
If you agree that TS is likely, then it probably is the mint of Thessalonika. I don't think there is any other possibility, but you could check on the
Wildwinds page for Constantine II.
The first two letters are normally, for this mint as for many others, SM = Sacra Moneta. Perhaps the first letter is too thin for an "S", but corroded coins are often misleading. Have a closer look and let us know.
If indeed it's Thessalonika, then the final letter should be Greek ─ that's to identify the "officina" (i.e. room) in which the coin was struck. Here, too, the corrosion may make it a little tricky. I suggest it's "B" (beta).
It does look like Thessalonika struck such coins while Constantine was still Caesar under his father, so that's promising.
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