That plate is a dissimilar metals plate so the bell housing of the transmission does not cause a reaction to the block its bolted on to. Also so they can close off the area of the bell housing with out making the motor casting larger. Believe most call it a "transmission block plate"
Any rubber grommet will fit will work. I think its usually used a peak hole to inspect items like the flywheel or flex plate with out having to remove the starter.
2001 ZX3 2.0L Zetec-
Hatch Nation #42
Going through Rebuild See "The TICK Build Thread" for details (http://www.focusfanatics.com/forum/showthread.php?t=289168&referrerid=62897)