It stands for Wally Byam Caravan Club International. The big red numbers you see on other Airstreams is the membership ID number of the owner. Ours, for example is 3021.
Do a search on WBCCI on this site, and you'll find out more than you'd ever want to know about the organization. It is international, and there will be a unit 'local' to your area that you can join. They will take care of your unit and International dues. You can contact the WBCCI offices at (937)596-5211 or at
www.wbcci.org to find out who to contact in your local unit.
Welcome!
Roger