In the early days, the 2 5/8 inch paper case was the "standard" 12-gauge shell. The 12-gauge 2 5/8 inch shell could be had with loads up to 3 1/4 drams of bulk smokeless powder pushing 1 1/4 ounce of shot. Circa 1907 all of our U.S. ammunition companies dropped 1 1/4 ounce loads from the 2 5/8 inch 12-gauge shells. From then on one had to go to 2 3/4 inch or longer 12-gauge shells for 1 1/4 ounce loads. The Remington Autoloading Shotgun, the Remington Repeating Shotgun, the Winchester Model 1897 and the J. Stevens Model 520 pump were all made for 2 3/4 inch 12-gauge shells. Our U.S. ammunition companies continued to offer 1 and 1 1/8 ounce 12-gauge shells in the 2 5/8 inch case up to WW-II --
FIELD 12-ga late 1930s top flap.jpg
FIELD 12-ga late 1930s front.jpg
RANGER 2 5-8 inch 12-ga 1 ounce.jpg
RANGER 2 5-8 inch 12-ga 1 1-8 ounce.jpg
RANGER 2 5-8 inch 12-ga Skeet Load.jpg
SHUR SHOT 12-gauge 2 5-8 inch #6.jpeg
In that the 2 5/8 inch shell was considered "standard" most boxes up to the later 1930s don't have the length given on them. If they were for the longer 2 3/4, 2 7/8 or 3-inch they are so marked. Same with the 2 9/16 inch 16-gauge and the 2 1/2 inch 20-gauge, no length marked on the box?!?
NITRO CLUB X20D7 Remington Arms - Union Metallic Cartridge Company Incorporated.jpg
16-gauge Nitro-Express 1934 to 1939 7 1-2.jpg