After Savage bought Fox, all their guns were supposedly chambered 2 3/4". If you sent them back to the factory for other repair work, they'd automatically lengthen the chambers for you, unless you specifically told them not to.
Per my 1936 Abercrombie & Fitch catalog, Parker had switched to 2 3/4" chambers as standard in 12-16-20 by that time. For more exact dates on when companies like Ithaca and Parker switched to the 2 3/4" 12ga as standard, you'd probably need to find someone with catalogs from the respective companies from that era.
However . . . some companies, even when they marked the guns as having 2 3/4" chambers, continued to intentionally short-chamber their guns. The reason: they'd found that the paper case mouth, opening into the forcing cone, provided some protection to the shot on its initial contact with the barrel walls. Thus reducing deformation and resulting in better patterns. But if a gun is factory marked 2 3/4", whether it measures that or not, then it would have been proofed to the higher standards of the 2 3/4" shells versus the old 2 5/8" shells, which were loaded to a lower pressure standard.
