I may be mistaken but the difference in price between a documented vs un-documented "No" gun is gonna be close nominal at most.
Like the difference in price between docs or no-docs on a "☆1" stamped NVA.
Essentially, the "M21 No" stamp IS the documentation.
Also. I wish I did, but do not have an early "No" stamped M21... not yet.
Good luck in your quest.