Earlier Rings didn't have serial numbers. From the mail I had from Mike, the earlier boats were usually tracked by owners, colours, design etc. Serial numbers only started getting used on mid/late 80.s builds.
If you do a search on here you will find a fairly lenghty discussion on the subject.
As a reference, mY Ring (a 21B) has no numbers at all.
The less time I have to work, the more I get done !!!