I've posted this before.
By the mid-1920s, Ithaca had beefed up their Flues doubles quite a bit. In the 1912 through at least 1915 Ithaca catalogues they give their smallbore weights as --
16-ga 5 3/4 to 6 1/4
20-ga 5 1/4 to 5 3/4
28-ga 4 3/4 to 5 1/4
In the July 1919, Ithaca catalogue --
16-ga 5 lbs. 14 ozs. to 6 1/2
20-ga 5 1/2 to 6
28-ga 5 to 5 1/2
In the December 1, 1919, Ithaca catalogue --
16-ga 6 to 6 3/4
20-ga 5 3/4 to 6
28-ga 5 1/4 to 5 3/4
In the 1925 Ithaca catalogue --
16-ga 6 1/4 to 6 3/4
20-ga 6 to 6 1/2
28-ga 5 3/4 to 6 1/4
At the time the two Ithaca No. 1 1/2s in question here were new, North American 20-gauge shells were offered with 3/4 and 7/8-ounce loads. When the 1-ounce, progressive burning powder, high velocity, load came out in 1922, Western Cartridge Co. put their Super-X load in their 2 3/4-inch FIELD shell. Remington with their Model 17 pump made for 2 3/4-inch shells followed with their 20-gauge Heavy Duck Load put up in their 2 3/4-inch Nitro Club shell. Winchester with a boat load of their Model 12 20-gauge pumps out in the world made for 2 1/2-inch shells stuffed the 1-ounce high velocity load in their 2 1/2-inch shells.
Leader 2 1 2 inch 20-gauge 1 ounce Du Pont Oval.jpg
Winchester continued to offer the 20-gauge 2 1/2-inch Super-Speed load up to WW-II --
Super-Speed 2 1-2 inch 20-gauge.jpg