OK, it's complicated.
First of all, it's not a mint mark, all of these are from the Constantine mint.
Krause tells us, these marks appear on the coins, but not what they mean, or exactly where.
They do not say that the marks correspond exactly to a regnal year of issue, which would have been my guess.
Here are my cheat sheets from the Krause catalog - you now know as much as I do.

To add another complication, the list in Krause for this coin seems different than the list in the Numista catalog page.
I have looked at your coin, the coin in the Numista catalog, and seven others that appear in recent Katz auctions, but I can't make out where the marks are. Have at it!
This is just the kind of thing that keeps numismatics interesting!
Jamais l'or n'a perdu la plus petite occasion de se montrer stupide. -Balzac