OK- your pictures are not the best (need overall pic, good lighting, LOTS of pixels so can enlarge)
Imman(ual) Meffert is listed in Journal-Verlag Schwend Hall GmbvH Schwabisch Hall 1979, volume 2. Was able to find listings for Immanual Meffert active in Suhl in 1846 and between 1870 and 1882. In other words, you have a very old gun made in Suhl, Germany WAY WAY over 100 years ago. This is a black powder gun, and probably should not be fired with ANYTHING. Short of finding a sister gun, you will not "find" parts for this one- these were guild made (hand made) guns.
If you want to see more of his guns, you can buy a copy of the catalogue of Meffert guns from the German Gun Collectors Association.