Parker, I believe made an A (the TOTW copy) and a B sized brass shell for a 10, and guns were chambered for one or the other, there was a slight difference in the specs

I don't have my Parker Story books handy at the moment, but I will confirm when I can if no one else comments in the mean time


BTW - my Parker ten will chamber the original Parker A size, as well as original Winchester and UMC 2 7/8 brass, so I am guessing the A was the standard chamber size