Yup, take-up wasn't national at 'A' first time round. Even in '64 there were still some outliers counties using three numbers and letters (my Mum had an early '64 Morris with no suffix).
Very few 'E's too I think, 'cos that was the half year as they moved from calendar year to Aug-July for registrations. (But only unit As by then of course . . . )
TEN 650A would be a very nice plate for a '63 machine, or even A650 TEN from the round-we-go-again later series.
The joys of the UK's numbering system . . . could it be the ULTIMATE anorak subject? PEN 15 anyone? Or H151 and HER1, a pair I saw on matching humdrum Austins years ago . . .