17 fire-resistant sealing plugs made primarily of rubber; sealing material, namely, gaskets, packings, sheaths, bushing, sleeves, and cuffs made primarily of rubber designed to prevent the propagation of fire, fluid, or gas along cable and pipe lines or ducts; sealing rings, namely, rings of rubber for use as pipe connection seals, and sealing muffs, made of rubber designed to prevent the propagation of fire, fluid, or gas along cable and pipe lines or ducts; flexible pipes not of metal