The 1925 Winchester Catalog 83, states the 16-gauge is made for 2 9/16-inch shells and the 20-gauge for 2 1/2-inch shells. I have a Winchester booklet, The Game, the Gun, The Ammunition, Form No. 980, which I can't find a date on, that states "The 20-gauge Model 12 is now chambered to handle standard 2 3/4 inch shells." From the 1922 Winchester pocket catalog through this booklet the Model 97 is listed as 12-gauge only. The big July 1931 Winchester catalog shows both the 16- and 20-gauge Model 12s chambered for 2 3/4 inch shells, and the 16-gauge Model 97 back, now made for 2 3/4 inch shells.
I too have a 16-gauge Model 12 story. When I arrived in the DC area in September 1975, I had Model 12s in everything but 16-gauge. I pretty quickly hooked up with a local restaurateur who had three great English Setters for some good Southern Maryland and Eastern Shore Quail hunting. I pretty quickly realized he played fast and loose with property lines, so to take my 20-gauge Skeet Gun out of harms way, I picked up a 1953 vintage Model 12 16-gauge 26-inch, Imp, Cyl. well worn but not abused, at Clark Bros. for $165 out the door. Things went well for about five years, then my friend hooked up with his big chested bar tender and his wife got the setters in the divorce!! Didn't shoot a Quail in Maryland after that.
