Well . . . to start with, you're talking apples and oranges when you throw Fox and Elsie into the mix with NID. Fox and Elsie were making short-chambered 20's and 16's prior to WWI, and they were sourcing at least some of their barrel blanks from Europe. There was indeed a change in the steel used in those guns, pre-war to post-war. However, I can't tell you how much of a difference that made in strength, comparatively.
NID's, of course, were all post-WWI guns, so you don't have quite the same factors at work. However, are you sure that the materials and dimensions were identical? Obviously, one dimension was not: chamber length. And what I do know is this: After the Super-X appeared, 2 3/4" shells were manufactured to a higher service pressure level than were the old, short shells.
And I also know--per an article by LTC Calvin Goddard, American Rifleman 1934--that American manufacturers proofed their 2 3/4" guns to higher levels than their short-chambered guns, because SAAMI standards were different: 13,700 psi mean proof pressure for the 2 5/8" 12ga; 15,900 psi for the 2 3/4" 12ga. Service pressures for the guns, respectively, were 9,500 and 10,500 psi. (I think those are actually LUP values, which back then were incorrectly expressed as psi.)
All that taken into account, it's also true that Savage, after they acquired Fox, would routinely lengthen short chambers on Fox guns sent to them for repair work. So you have "evidence" pointing in both directions. Prior to WWII, none of that was a problem, because there were in fact more short shells being manufactured in this country than there were 2 3/4" shells. But if it were my gun, I'd exercise greater caution in ammunition selection if it originally had short chambers than if it came straight from the factory at 2 3/4".
