Stan;
You are of course correct. I can't say for certain about the 21, but for the others, they weren't "Magnums". There were also 3" 20 gauges long before the Magnum version. Nash Buckingham wrote of his father have a W W Greener hammer double in 12 ga with 3" chambers. He used it on ducks in the 1880s as I recall but only loaded it with an ounce of shot.
Stevens had a 20 gauge pump with 3" chambers which preceded the Win model 12 seem to recall perhaps was the model 10. Up to around WWI virtually all the gauges could be had with chambers from 2" up to about 3" in 1/8" steps on order, with a few 1/16" increments thrown in for good measure. 2 9/16" would become a standard in the 16 & was I believe also a standard in the 14, if anything can be called "Standard" in the 14.
I have a circa 1890 10 gauge Lefever with 3" chambers, never to my knowledge a Standard for that gauge. Standards for the 10 were for the most part 2 5/8", 2 7/8" & the 3" mag.
As you said though these early odd lengths were not for the purpose of increasing the shot load, more for better wadding lengths with standard shot & powder loads.
