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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.