Just taking a swing here since there are serious researchers on the site.
I am about to wrap up "version 1" of my project of gathering the French military correspondence for Armée du Nord/Moselle in 1815. I have 1,500 transcriptions for June alone. Stuff from SHD is necessary, but generally well known. (I have 5-6 thousand images from numerous French archives/libraries as well. My project focuses on the correspondence.)
The key to the full understanding is contained in the private collections and the unseen registries. This material is what changes the conventional history. The history is largely based on Grouchy's Registre du major-général (a copy), Reille's registry, and of course Napoleon and Davout's correspondence. (If someone mentions Vandamme, then understand they really are not very serious about the topic.) I have 4 additional registries.
I have identified approximately 400 additional pieces in private collections. Assume far more than that.
And that is what I seek. And I have $$$.
If you have also researched this topic or are active in auctions, I am interested in collaborating and sharing.
Write me: stephen.beckett@comcast.net