Birmingham car registration letters.
(Admins, you might want to move this elsewhere, please feel free so to do.)
The first letter is just an index. OF 1 to OF 9999 was followed by AOF 1 to AOF 999, then BOF 1 etc.
The Birmingham identifiers were: - O, OA, OB, OC, OE, OF, OG, OH, OJ, OK, OL...