Watch List notes:

Probus. A.D. 276-282. AE antoninianus (22.66 mm, 3.98 g, 11 h). Siscia mint, struck A.D. 280. IMP C M AVR PROBVS P AVG, radiate-helmeted, cuirassed bust left with spear over shoulder and holding shield / PAX AVG, Pax standing left holding olive-branch and transverse scepter; in field, P; in exergue, XXI. RIC 709; C 406. EF, Hints of silvering.

this is a less common form or title / obverse legend for Probus (just "P", not "PF") - RIC notes it as scarce