Not a Snider, but the same idea. It is a "tabatiere" ("snuffbox") conversion of the French Mle 1857 rifle musket into a breechloader. The conversion was done c. 1867 by the French to extend the useful life of their old muzzle-loaders. Later, the guns were sold to Belgian entrepreneurs who reamed them to make shotguns and sold them throughout the world, including the U.S., where they sold for $3 or so. They show up in the old catalogs; there must have been a lot of them as they still turn up very often.
They have some curio interest, but seldom bring much in the way of money, usually running under $100 even in good shape.
Jim