Become a Full Member and get the official Linkage Tour and the official Starting Aid. This information alone will pay for your membership
Pawel is the part number genius of this forum!
the gaskets are a black rubberized cloth. very thin. The closest thing I can think of is trucker's tarp material. I would use a thin sheet of nitrile. 1/16" or so. Since the central hole already has a rubber seal sealing the rod I would imagine the function of the black rubber is either to prevent squeaks if there is any movement between the pedal/ booster assembly and the firewall or to add "Grip" to prevent movement.