I read the author’s post in Zfrontier and here is some clarification for those who didn’t quite understand the gasket structure like myself.
The rubber gasket is actually not shown in the exploded view. The metal perimeter part that goes between the plate and the bottom piece is not a gasket. That part goes on the bottom piece and provides a structure for the gaskets to sit on. The part is optional, meaning if you want gasket mounting, you would have to purchase that piece separately or you can opt for top mounting. Also, that piece is available in brass, polycarbonate, or aluminum.