From the Parker Gun Collectors Forum:
http://www.parkergun.org/forums/forum1/2164.html Parker "borrowed" English chamber measurements (for paper shells) and manufactured 12ga and 16 ga 2 1/2" chambers until about 1895...then the 12 ga went to and continued with 2 5/8"chambers (16ga went mostly to 2 9/16" ) for a long period...possibly until near the end of regular Parker production...about 1934. By 1930 most (but not all) shotshell and gun manufacturers agreed to 2 3/4" chambers for field and target guns. As far as I can determine, 2 5/8" shells were manufactured at least until 1940. The 1945 Stoegers Shooters Bible lists Xpert and Xpert Super Skeet loads, Ranger Field loads, and Leader Staynless loads as being available in 2 5/8" 1 1/8oz. By that time all Super-X, Super Speed, Leader Super Speed, Ranger Brush Loads, etc were 2 3/4" 1 1/4oz. Original chamber length is a strong clue as to the correct MAXIMUM shot load.
I don't know what to make of the 1920 "2 1/2" Standard Shotgun Chambers drawing" (p519 TPS). Some information is obviously missing. In original 12ga chambered guns (frame size #2 and smaller), a 12ga .798 chamber gauge will stop most commonly at either 2 5/8" or 2 3/4". Why does a .798 gauge not stop at 2 1/2" as depicted in the drawing? The difference due to chamber taper is of course, negligible, but why would Parker make a "shop drawing" and not use it? Why was this 2 1/2 inch "standard" adopted in 1920 when Trap Guns were standardized at 2 3/4 inch chambers much earlier? Is there documented evidence to validate the "better gas seal" claim? Is there "short chamber" patent?
In the recently published book "The Parker Story" the Remington vintage specification sheets on pages 164 to 169 call for a chamber 1/8-inch shorter than the shell for which it is intended. I have a 1930-vintage VH-grade 0-frame 20-gauge and its chambers are 2 3/8 inches intended for the old 2 1/2 inch shells, eight years after the 20-gauge Super-X shell was introduced in a 2 3/4 inch case!!
New subject: Why would manufacturers continue to make 2 5/8" shells 15 years after they had adopted a 2 3/4" standard?
Presumably because 2 5/8" shells were designed to work in 2 5/8" chambers with a maximum 1 1/8oz shot load AND those shells worked equally well in 2 3/4" chambers. If there was gas leakage around the overpowder wad or in the forcing cone, no one was the wiser...or complained apparently. We shoot 2 3/4" shells in 3" chambers today (ex Ruger Gold Label) and no one raises an eyebrow. Conversely, 2 3/4" 1 1/4oz loads were NOT DESIGNED to work in light guns with 2 5/8" Chambers.
