So I'm looking at a 1915 catalogue from the Edw. K. Tryon Company, the large, well known sporting goods distributor in Phila. On just one page there are (33) factory-loaded 12-gauge "high brass" 2-5/8" shells listed, with respected trade names like:
Leader, Arrow, Ajax. The named shells were loaded by Winchester, U.M.C. and U.S Cartridge with Bulk and L&R dense smokeless powders.
Wow! There must have been many guns out there with 2-1/2" chambers intended for 2-5/8" shells ..... as per that 1/8" short convention we read about so often.