I am not sure of why the trigger guards were shaped the way they were. I have never seen any particular documentation around it but have not looked really hard either.
As for other sources and historical investigation, there are some books on the topic.Casimir Lefaucheux, arquebusier: 1802-1852 by Gérard Lautissier
It's a bit dated but still probably the best look at Casimir Lefaucheux and his contracts and patents and etc. You should be able to find it cheaper than this too. La saga des fusils de chasse Lefaucheux by Jean-Claude Mournetas
This book has less original research but is maybe an even better overview of the evolution of these guns.Systeme Lefaucheux: Continuing the ...he American Civil War by Chris C. Curtis
is an obvious one to have. It goes into a little bit of history and more so covers a very broad look at everything pinfire related. Plus it is in english!
On the pinfire topic it would be impossible to not include the books by Guillaume Van Mastrigt as well. He has been active on this forum too. His books detail the life and guns of Casimir's Lefaucheux's son, Eugene Lefaucheux. Eugène Lefaucheux ou L’apothéose de la saga familiale: 1832-1892 by Guillaume Van MastrigtLa Production des Modeles d’Eugène Lefaucheux by Guillaume Van Mastrigt
And for me, I love original source documentation and have been picking up original newspaper articles and clippings and early technical journals. One of these that I picked up has a 42 page article comparing and contrasting The new breech-loading rifle made by Joseph-Alexandre Robert and the one made by Casimir Lefaucheux
. It is definitely worth readying through my article on it: https://aaronnewcomer.com/document/...-and-the-battle-of-robert-vs-lefaucheux/
Plus there are many more books touching on different manufacturers and types of guns.I have a couple dozen in my library.