Original Wilmot's Patent .58 caliber Expansible Tompion. Hard to find piece! The metal parts are brass, fitted to an expanding rubber plug, covered with wool cloth. Turning the serrated top turned a screw thread that squeezed and expanded the plug to tightly fit the muzzle. Missing brass cover piece that separates the top knob from the wool. See picture and full write up in Volume 1: GUN TOOLS: THEIR HISTORY AND IDENTIFICATION BY SHAFFER, RUTLEDGE AND DORSEY.