Those numbers are given to you when you join the club. I just bought my AS, and the owner had two trailers, both with his number on them, the club logo and stars indicating how long they had been in the club (20 years). Of course he took them off before I came to pick it up. Now when I join the club I'll put my numbers on, and when other club members see us in a park or on the road, they can look up our number in the club book, and find out where we're from and who we are. Kind of cool, really.
Good luck, I hope you get your airstream!