I'm no expert but have owned and looked at a few Trapdoors but never seen a breech like this... Not counting the chunk of wood missing... That ledge or step on the rear of the receiver I've never seen before and there's no serial no. The breach block is an M73 and it has what looks like a M79 rear sight.
The barrel is about 30" long... Could this be a Cadet rifle?
Stock has been cut down and the cartouche is dated 1889.