I'm purdy sure this would be yours. (The Mle 1822/67T.bis Tabatiere conversion)
It seems that these Tabatiere rifles had a bad reputation do to their large caliber and ineffective ammo. As the French started to loose the French-Prussian war, they decided to transform the large amounts of M1822Tbis and 1842T rifles and even some AN IX flintlocks they still had in stock to the Tabatiere system. They were used to armed the "Garde National de Paris" and the "Francs-Tireurs" to defend Paris.
The bronze blocks where easier and faster to produce. Lots of them are transformed by Paris gunmakers like mine "Ets. Mignon et Rouhard, rue Oberkampf a Paris" (MR and the rear end of the breech block). After the French-Prussian War they were put out of service.
The bronze receiver and breech block, with the forward lock, are the notable distinguishing
charecteristics of this model of Tabatiere
It would go with your area (Maine) as well! I would think.
There was others conversions made but not like this. Snider made one for a enfield musket.
But nothing like that breech in your pic.
Take Care,
Pete,
Geeze Pete,
I greatly appreciate the work on that.
So if I understand this correctly, originally the gun was a muzzle loader. Then converted to breech load by a company other than the manufacturer.