Unlike many classes, the 'A' Class often has a personalised sail number in several national associations. The GBR association operates such a system whereby you are allocated a number of your choice, providing it is available, and that remains your number until you leave the class. You simply take it with you when you change 'A' Class cats.
New registrations cost £15 and there is a form here
Please see the current list of number in use. You are then free to choose something NOT on this list.