Capt. E.C. Crossman would be a credible source Bob; “Skeet Gun Patterns” in August 1936 Hunting & Fishing
“Improved cylinder is the greatest degree to which a plain barrel should be opened, this being not less than .004”. The finest example of this at present is the Winchester No. 1 Skeet boring, which has about .004 choke at a point 3” from the muzzle. The muzzle section then becomes larger…until finally the barrel at the muzzle is about …0.750” instead of the normal 0.730” of the 12. This is a relieved muzzle or bell muzzle, originated by Ithaca years ago.”

Walt and Researcher have previously posted the Ithaca engineering drawing.

I didn't find it in the WRAC drawings
http://library.centerofthewest.org/digital/collection/WRAC/search/searchterm/winchester%20model%2021

I've got the 12g M21 barrel drawing, but it does not specify the chokes

https://sites.google.com/a/damascusknowledge.com/www/home