Some amount of work and a great lot of information !.
I notice a couple of typos in your 'chronology' part as in 1872 Anson's Patent should read 3791 and 1877 A & D hammerless patent should read 1875.
Re missing serial numbers - it was quite common for gunmakers to use a 'block' of numbers and miss out quite a few thousand over a period. For example record books might start at 1000 instead of 1. Whether this was to appear to have greater production than the actual or to ddifferentiate between types or something is hard to know.