I believe that the higher grades, especially those with ordnance steel barrels had the longer chambers.
My 1907 vintage 12-gauge CEO-Grade has the "standard" 2 5/8 inch chambers. The later vintage hang-tags that have surfaced all state the 12-gauge guns were targeted with UMC NITRO CLUB Load X8 with 3 drams of bulk smokeless powder or 24 grains of dense smokeless powder such as Infallible or Ballistite pushing 1 1/4 ounce of #8 chilled shot.
The interesting thing is that between their July 1906 and their July 1907 catalogs, Union Metallic Cartridge Co. quit offering any 12-gauge 1 1/4 ounce loads in the 2 5/8 inch shell. From the July 1907 catalog onwards 1 1/4 ounce 12-gauge loads were only offered in the 2 3/4 inch or longer shells.
This is actually from the 1910 catalog. I can't get a clear enough picture from the PDFs I have of the 1907, 08 or 09 UMC catalogs for posting. This is a scan from an original catalog.
