Thank you, Samuel. I do not have the Schwing book on the M21, and I believe what you have stated. If I were a serious M21 collector I might want to own a copy, and he is a well-known authority on Winchesters.
My two "anchor points" in my gunning library are not quite as accurate as Schwing- (1) I have a copy of a WRA Salesman's Handbook, 1938- and on the section detailing the M21, the pages state the minimum net weight of a finished M21- in 12-16 and 20 gauges- with varying barrel lengths and other factors: recoil pads, style of forearm, rib styles, etc. I think it is safe to say that the M21 of that pre-War era was "over-built" for strength, just as was the M12 Heavy Duck Gun, which came out about 1936--
I did go to my copy of Gene Hill's "Shotgunner's Notebook" and he touches on M21's on pages 84-85: "It's often a personal evaluation, but, for example, I don't find that something extra in the old Parker 12 bores. They strike me as a touch clubby. Most 20 gaughe Model 21's are a bit heavy, while most 16-gauge 21's are almost perfect, using the classic standards."
It was from the late Gene Hills' comment that I inferred that the std. M21 in 16 and 20 gauges shared the same frame size. No big deal.
