The letters refer to the different profiles of the swamped barrels. You can't call it a 15/16 barrel since it's outer dimensions change so they refer to them as "A", "B", "C" and "D". "A" being a slim profile for say a .40 cal and a "C" or "D" profile might be for a .54 cal barrel. The below website for Rice barrels has some drawings that will help. Easier to see than to explain. Hope that helps some.
Packdog
