In the early days brass shells were available in two sizes for at least some of the gauges. The A sizes were made to fit standard chamber dimensions of "Paper Shell" chambered guns. The B sizes were for a gun having a smaller chamber designed for use of the brass shells in a standard size bore. Some guns were also built with the intention of using the A brass shells & had the bores oversize to accomadate the larger wads. I do not have dimensions for a 14B chamber but would think it smaller than the given dimensions.
I do not know if there was ever a "15GA" shell per say or not, however the British proof charts included all sizes from 1 to 50 inclusive with letter sizes above going up to 2.00" dia in 1/16" steps as I recall & below 50ga in odd ball sizes carried to two decimal places giving .010" increments from .45cal (51.05ga) down to .30cal (172.28ga).
The German proof law was "Passed" in 1891. It then took a while to implement it & don't recall exact date, but think Pete probably has it nailed. That sounds about right. In the interium guns completed & ready to sell were given a Crown over V mark indicating they had been approved to sell but were "Un-Proved". The tables I have seen of German sizes followed the British exactly.
