I also have a M 21 Serial number slightly after yours which frame dated 1941.However after receiving the factory letter from Cody it wasn't actually shipped until July 1947.
There is also a fairly complete article in the Shooting Sportsman by Mr. Schwing of Jan-Feb 1998 which would also be helpful. Lots of study and browsing should be done before purchasing so that you will know as much as possible when you make the purchase.