Ian, is "PO" an (old) Chichester registration? I have "nnn PPO" on my '05 4/4.
DaveW's link has the answer - "PO" was West Sussex (Chichester) until 1974 and then Portsmouth up till 2001. Now Preston, apparently. A useful document, thanks for posting.
I have a very old and well used copy of
Where's It From, When Was It Issued by Noel Woodall, which gives the date of issue of all letter combinations from 1903. Particularly useful for pre '63 registrations.
~iw
