Original posters gun looks like an early A&D 1875 patent with the rounded ends to the cocking levers. Argo I think the 1877 mar 24th Advert is in fact for their tumbler blocking system where a cross bolt blocks both tumblers.
At the time of 1875 patent London houses were left somewhat in disarray as all they had to offer were hammer guns. A number of arguments were made against the A&D System ranging from "it looks ugly, like a dog with no ears"
to" its dangerous, you cant tell whether its cocked". To combat these any number of weird and wonderful devices were introduced to overcome problems that didn't exist.