Brian: Up until about WWI the standard 20g load in the U.S. was 2 1/2" with 7/8 oz. shot and 2 1/4 Dr. Eq. Bulk or Dense Smokeless powder at 1155 fps; 7/8 oz. with 2 1/2 Dr. Eq. at 1210 fps thereafter.
It was not until 1923 that the new Super-X and Nitro Express 2 3/4" 20g shells had 2 3/4 Dr. Eq. of progressive burning powder and 1 ounce of shot.
I don't know when Parker begin boring longer 20g chambers; Hunter Arms did not until 1936.

However, Edwin Hedderly, editor of Western Field magazine promoted small bore shotguns for duck hunting and received some fabulous Parkers in exchange for ads; including a 32" 20g A1 Special ordered April 26, 1912.
The Widgeon Duck Club in central California obtained 10 special order, 3" chamber 20g Parkers. Researcher has an early Peters Cartridge Co. 3" 20g shell loaded with 2 1⁄2 Dr. Eq. "E.C." and 7⁄8 ounce of No. 4 chilled shot.

Forest & Stream, February 27, 1909
“Small-bore Guns and Loads”
https://books.google.com/books?id=nE...AJ&pg=PA337&dq

T.H. Grant, Forest & Stream, December 1915
https://books.google.com/books?id=lR...J&pg=PA711&lpg

Edwin Hedderly, Western Field editor after July 1910
Arms & The Man 1915
https://books.google.com/books?id=BX...J&pg=PA158&lpg
Recreation 1917
https://books.google.com/books?id=4u...AJ&pg=PA97&lpg
Outdoor Recreation 1919
https://books.google.com/books?id=BX...J&pg=PA158&lpg

Thank you drew.

Heading south to the red dirt plains to hunt quail in a couple weeks with Bert and the new pup. Nothing better than to get them on real birds at 17 weeks old, that can make a bird dog.

I’ll grab the old D 20ga and boxes of Win, Fed, Rem and RST 7/8 oz shells so I don’t have to stop at the local hardware stores. Anybody see any birds here?