I used to have a 1940 WM20 but sold it as finding all the special 1940 parts was very expensive and with already owning 12 WD Royal Enfields it wasn't really in keeping with the collection.

Your bike has been modified for the Volkes air filter system which mounted on top of the tank, hence why one side at the rear has been removed. Are you aware of Henk Joores website and forum? There are people on there who have quite a few of the original Key cards and maybe able to give you some history of your bike. I think you can also ask the MOD for it's record too but more than likely it would just be the basic details of contract details and disposal date.

Do lots of research before you buy any spares as the post war M20 was a little different, anything with a YM or ZM20 prefix is post war.