Your bike is a 350cc OHV WD production from WWII. It may be possible to find out the exact date it was made from WD contract info but generally speaking it's like the old WD BSA M20. They arrived at a standard WD specification and built to that regardless of exactly what year it was. Nevertheless there can be a lot of minor variations caused by wartime shortages. These variations don't alter the overall bike but can be a puzzle at times.
Cheers, Bob