The original bolt knobs of the 1889 and 1911 rifles were made of hard rubber, also vulcanite or ebonite, it's a hard, moldable, polished dark colored (ranging from brown to black) early rubber.
Vulcanite was produced by adding sulfur to vulcanized rubber. Corrosion makes the screw that holds the knobs to expand causing the rubber to crack, later replacement knobs were first made of bakelite and now they are made of plastic with stainless steel screws.
Original vulcanite grips often carry the serial number of the rifle.
The same vulcanite was used to make the grips of the 1878 and early 1882 revolvers and the Vetterli 1878/87 bayonets that's why we find some of these with brown grips.
Guisan.
