Well, I heard that originally it was POHM's, meaning Prisoner Of His/Her Majesty.
"If you check wikipedia it goes on to say that:
Some proponents of this theory claim that upon arrival in the country they would be given a uniform with "POHM" or "POME" emblazoned on the back, which apparently stood for Prisoners Of Maidenhead Prison but there are no images or examples of these uniforms".
So another prison added to the list, but I cannot see how Maidenhead prison would give the initial POHM.