I believe that the disc belonged to a member of the Royal Navy.
The P, for Portsmouth, being added in 1934, and the X to show that the man in question was in receipt of the (lower) post 1925 rate of pay.
The JPEM stands/stood for Junior Provisional Electrical Mechanic.
I had to google to find out exactly what JPEM stood for, and found this interesting account of a JPEM who took part in the D Day landings, carrying American troops to Utah:
https://www.combinedops.com/HMLCT%20795.htm