Maybe of use to some.
Note, I believe that Letter sizes were only used for punt guns, whereas Number sizes were used for punt guns, bank or rail guns and pure shoulder guns.
English Shotgun Bore Sizes.pdf (186.2 KB)
When I say bank or rail guns these were technically shoulder guns but were used rested on a bank side or a fence rail and were not used standing and swinging at the target although some were carriage mounted.
People referred to punt guns by the measured sizes i.e. 2 inch (2.000") or 1 1/2 inch (1.500") rather than the letter bore size.