Was it just engraver's discretion where to put PM on reverse or obverse? Could this deviation from common types with PM on reverse indicate another mint location?
Also, do you happen to know why PM appeared only on TR P XVIII to TR P XX denarii of Aurelius, and never appeared on his later denarii until TR P XXXIIII?
No, not engraver's discretion,
nor indication of a different
mint. The movement of P M from one
side of the coin to the other was in all likelihood a chronological change indicating a new phase of the mint's production.
Notice that while P M was omitted from Marcus'
denarii from TR P XV-XVIII, it was included on
his bronze coins of the same period, after a short initial issue without P M probably produced before he
had officially been elected to that position.
I don't know why P M was omitted from Marcus' entire coinage after TR P XX.
Commodus continued the same anomaly: P M didn't appear on
his coinage until 184 AD, four years after
his accession as sole emperor in 180.