You're probably looking at a butt plate cast from 'gutta percha'. It's a commonly used material to cast many different things in the last half of the 1800's and the early 1900's till plastics (like Bakelite) took over.
The material is made from the sap of the gutta percha tree.
Some call the material 'hard rubber' when refering to pistol grips and butt plates cast from the stuff,,but I don't know if they are one in the same materials.