Apparently only three other specimens are known, two of them with a clear S at the end of
EXERCITVS.
Bastien 359, pl. XI,
Milan.
Exergue damaged, but reading [AQ]S seems probable. Last letter of EX - ERCITVS off
flan, so EX - ERCITV[M] can't be excluded. From different dies than yours.
Bastien Supplement 359a, pl. S V, BM.
Mintmark mostly off
flan, read AQP, but AQS looks more likely. ERCITVS on right fully clear. From different dies than both yours and the
Milan coin.
Ditto 359b, same pl. S V, Auctiones 10, 1979, 799.
Mintmark clearly [A]QT. ERCITVS on right fully clear.
So your EX - ERCITVM indeed appears to be a new
legend variety, unless that is the correct reading of the
Milan specimen too!