Does anyone recognize this Mintmark?

I have this Constantine I / VOT XX AE20mm., 2.95gm., (similar to) RIC VII 85, coin but I cannot determine the mintmark. I am hoping that someone, with more expertise than I have, might be able to pick it?
(TR, ARL, AQ, TIC, ROM, SIS, TES, HER - or anything else?)

 

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Try to clean the exergue with a tooth brush. Take your time (maybe days). You can also use toothpicks without harming the coin at all.

 

After much soaking and scrubbing we have . . .

 

TT?? Maybe Ticinum