Postumus AR Antoninianus. Cologne mint, 268 AD. IMP C POSTVMVS P F AVG, radiate, draped and cuirassed bust right / PAX AVG, Pax standing left with branch and sceptre. P in left field. RIC 318,
This is one of the series with P in the field. I am not comfortable explaining the purpose of the P. If it were a workshop, there should be some S or T coins as well but there are not. It may mean simply Postumus which seems odd except for the coins of the next Gallic Emperor Victorinus which have a V in the field. Ideas anyone?