I've taken a look at this and it seems that the old Times New Roman font makes "RAFVR" look like this:
RAFVR
Most journals of the time used this font (newspapers and Flight) and when text is transposed from these onto the internet the "F" become "P"s. Hence JamesDevice's RAPVR in his first link and RAFVR in his second.
It should read RAFVR or Royal Air Force Volunteer Reserve - distinguished from the regulars by a VR on the lapels of their No1 uniform.
CPL Clott