The great bulk of 2-frame 12-gauge steel barrel Parkers have 2 5/8-inch chambers, intended for use with 2 3/4-inch shells. From the time the Parker Spec. Steel barrels were offered for Quality GH guns in 1907, the heaviest loads our North American ammunition companies offered in the 2 5/8-inch 12-gauge shell were 1 1/8-ounce. For 1 1/4-ounce loads one needed to go to 2 3/4-inch or longer shells. The heaviest 12-gauge loads being offered were 3 1/2-drams of bulk smokeless powder or 28-grains of dense smokeless powder such as Infallible or Ballistite pushing that 1 1/4-ounce of shot. Pressures of those loads were a bit higher than SAAMI specs for 12-gauge 2 3/4-inch shells. Circa 1922, the progressive burning smokeless powder, high velocity, 12-gauge 1 1/4-ounce loads were introduced. While they moved the shot load out faster, they were actually lower pressure than the old style specified loads.
Pictures of hang tags I've saved are more confusing than enlightening.
107705 01 12-gauge, 2 3-4 inch, 1 1-8 ounce.jpg
108998 12-gauge, 2 3-4 inch, 1 1-4 ounce.jpeg
125594 12-gauge, 2 3-4 inch, 1 1-8 ounce.jpg
214724 01 12-ga 2 5-8 inch.jpg
Some give the powder load in grains when the powder stated is bulk?!?