Need help on the British acronym P.C.

[ktanguay@Accurate.COM (Ken Tanguay)]

Fellow Who-sters,

This is more of a Beatles reference.  In the song "Maxwell's Silver 
Hammer", they sing "P.C. 31, says 'We've got a dirty one', Maxweel 
stands alone...".

I was watching the Beatles movie "Help!"  and they were introduced
to Bahamian Police officers as "this is P.C. 5, this is P.C. 6, etc".

I know that it has somekind of COP (Constable on Patrol) reference,
but what does P.C. stand for?  Principal Constable?  

English Who-sters, please help out.

Thanks,

Ken Tanguay
ktanguay@accurate.com