I seriously doubt that the 2 5/8" chambers had anything to do with made for America. At least as early as W W Greeners ninth edition of the gun & its development (1910) he stated that the "nominal' 2" chambered 12 gauge had chambers from 2 9/16" to 2 58" length. I do not know how much earlier this practice was used. I believe that only very early British 12 gage guns will be found with actual 2" chambers. The European standard of 65 mm s 2.559" or only .0035" shorter than 2 9/16".

Also, 2 5/8 was an American standard, not "THE" American standard as late as 1929. Boh L C Smith & Lefever Arms Co had standardized on the 2 /4" shell in the 12 gauge long before this date, & perhaps others as well.


Miller/TN
I Didn't Say Everything I Said, Yogi Berra