It's a type officially known as a "Post Reform (replacing the Antoninianus) Radiate Fraction".
No, the coin is an
antoninianus (pre-reform). Trimalchio's ID is correct. Post reform radiates are covered in
RIC volume VI and never will have the
XXI. I wish I could say that all coins with
XXI are pre-reform but there are a few of the larger
AE1 folles that used this formula post-reform so we will have to settle for there are no
AE3 size coins with
XXI that are post reform. This example is quite typical of its
RIC number but unusual compared to most coins with
XXI in the way those letters are splayed out across the
exergue. Your example lost the I to the edge of the
flan. My example is below. Below it is the corresponding
Diocletian.
For what it is worth, I find interest in the fact that
Maximianus and
Diocletian used some more interesting reverses with
Hercules and
Jupiter on pre reform
antoniniani while the post reform
types were more likely to be generic
Concordia and Genio. Some of the later pre-reform coins also used what I would call the lesser
types so it seems the Imperial pair backed off from their close ties to their
patron gods a
bit before the reform. I have no idea why. Was there a
Christian in a position of influence at the
mint? We do see the reappearance of
Mars and
Jupiter a few years later but it seems old that the old gods took a hit about the time they founded
the tetrarchy.