Hi
I'm not sure about this. It isn't totally clear and it's not in RIC.
S doesn't follow the structure to be in exergue as ‘2nd’, as the officina marks that appear in exergue are all Greek, not Latin. In Greek, 2nd would be B, which is in the list. The Latin officina letters (P, S, T, Q) are all in the field with XII in exergue.
A Greek number that looks like S is ς, which is used for 6th officina. That might work if there was also Є (5th).
But it could be a corroded B? https://www.wildwinds.com/coins/ric/aurelian/RIC_0386_B.jpg