Thanks but alas l am not copping.
With the Reverse yoke model it seems that only the year 1941 is used to describe it , not 1940-42. So a very brief experimental period for the manufacture of the 501 then?
I did try and find out years ago about the csf war denim but drew a blank. Maybe it's a secret?
Edit. Also looking at your above query, traditionally levis 501 had 3 button fly plus waistband button until c.1936, when they gained a fourth fly button.