You might trace the name of the original or subsequent owners of Sparky by exploring either the 21 manual (assuming this contains queries and answers extracted from the newsletters) ,the newsletters themselves or old members handbooks.I am not sure who you would apply to for this - the membership secretary ? - or you could ask any other longstanding members who have owned a 21 from a an early date.
Peter
PS If it came with a trailer you could try the manufacturer,armed with the serial no.