Those numbers represent membership in the Wally Byam Caravan Club International as John mentioned in his post. Becoming a member of WBCCI is one good way to really get involved in Airstreaming (IMHO). In one of your other posts, you mention the Pendleton Round-Up. In the past, there has been a WBCCI Rally held in conjunction with this event; and it is my understanding that it is one of the premiere WBCCI events on the West Coast. It is quite possible that your coach's previous owner was a WBCCI member.
There are all types of Intra-Clubs in the WBCCI to address the hobbies and other interests of WBCCI members such as:
Vintage Airstream Club - - members who either own or are interested in any of the Airstream products that are 25 years old or older.
Computer Club - - members who are interested in personal computers.
Radio Club - - members who are interested in HAM and or CB radios.
Free Wheelers - - members who travel alone in an Airstream product.
You might find the Vintage Airstream Club of interest as I believe that you mentioned in another post that your trailer is a 1976 Sovereign
. You can learn more about the Vintage Airstream Club at:
Vintage Airstream Club
When one joins the WBCCI, one of the benefits of membership is a copy of the membership directory. This directory lists all members alphabetically as well as by membership number. In addition, there are listings by state for campgrounds; and a complete listing of Airstream dealers. There is also a section with a comprehensive calendar for all WBCCI scheduled activities by region for the entire year.