The grounds for this interpretation are:
(1) The prominence given to P M by its inclusion in the
obv. legend, particularly in the sub-issue where it is separated from the rest of the
legend and placed right
side up below the
bust. The last time P M meaning
Pontifex Maximus had been included in the
obv. legend was, I think, under
Marcus Aurelius.
(2) One of the
rev. types of the issue is the only one of Philip's reign that specifically commemorates
his settlement with the Persians:
PAX FVNDATA CVM PERSIS.
(3) The early date of this titulature with P M is confirmed by its appearance on coins of
Viminacium of Philip's first year, AN V, and on what appear to be
his earliest coins at
Antioch in
Pisidia.
(4) Inscriptions confirm that Philip indeed assumed a
victory title for
his Parthian settlement early in
his reign, but dropped it later: three milestones calling Philip Parthicus
Maximus, and a dedication calling him Persicus
Maximus.
For illustrations and references see
Pink,
Antioch or
Viminacium?, Num. Chron. 1935.