Hi Jon,
The 20 extra spokes were for the belt rim. For some bikes (like early Triumphs) the belt rim was attached to the exiting spoke, while other used the extra short spokes, and sometimes both methods were used.
Believe it or not, chain drive dates from the earliest days of the motorcycle c1900, but it was not popular in the early days as it was considered harsh. As chains improved, and various cush drives were perfected, chain drive became more common. Belt final drive had pretty-much disappeared by the mid 1920s.
If you post a photo of your wheel and the hub we can probably identify it.
Cheers
Leon