AUS 019 said...
Actualy its AUS, ISAF the international racing body for all sailing set these more then a decade ago and anyone competing in grade A racing has to have a national prefix before their numbers to race. As wave sailing doesn't fall under ISAF people have kept using the old numbers and no one really cares, for all its worth you could coompete with your name on your sail.
I think that it was more like 3 decades ago that the IYRU as ISAF was then known set a standardised 3 letter country code. Australia became AUS from KA Britain GBR from K. Possibly windsurfing only followed suit later, but certainly International classes have used the 3 letter codes for ages.