If memory serves me correctly, the British "P" labeled knobs control the
panel light brightness via rheostat. Many are disconnected since even
at their brightest the panel lights live up to the "Dim, flicker, and
off" reputation hard earned be Lucas.
If the knob twists without obvious detents and does not go in and out, I
think you have solved the mystery. If it is a pull, push switch, then I
would look for some opening through the floor to appear/disappear.
Maybe the "P" stands for what many of us need to do after a couple of
pints at the pub.
Regards,
Bill
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