This uniquely shaped face plate cover is designed to give you an extra 1-1/2" inches of water depth when bathing. It works by blocking the bottom portion of the overflow exit holes until the water level reaches at least halfway.
The benefit of this style of cover, is that it provides you a deeper bath, but still allows for full volume exit once the maximum water level is reached.
This replacement faceplate cover is meant to replace your existing overflow hole faceplate, and is listed here by itself. This package includes only the faceplate, and attachment screw.