Dad; What's a HAF?

       As a few might know by now I still "collect Car numbers".  "When we were very young" (© A. A. Milne 1924) there were so few Cars in, or passing through, the Village that we actually "collected" them on Paper with Pencils.
       THAT I have not done for well in excess of seven decades - BUT - as you may have noticed, some still catch my eye.  Like e.g. there's a bloke called John Bogie whose Car number actually reads:-
BOG 1 E I've seen it.
       Likewise B19 TUP  &  A11 HEN & others; a lad I was at school with drives around in B10 HAY which, logically, (for me at least) brought forth the thought seen in the Title.