OK, the P4 was indeed the standard platform compass for bombers in the early years of the war, it sat in a gimballed mount just ahead of the control column. The P10 was almost the same to look at but had a slightly different and improved magnetic arrangement. Fighters appear to have used mainly P8 and P11 compasses. Differences mainly concerned the relationships between mountings and aircraft flight attitudes.
As an aside, it seems that "P" compasses sat Platform mounted, in gimbals, with the card face up while "E" types were panel mounted instruments with the compass card 'vertical' - that is, marked on the Edge so you read them vertically.