Riser plate for flush mount base plate. (Required if you are not routing the flush mount base plate into your bench.)
If you are not planning to router your bench to install your flush mount base plate (i.e, mount it directly to the benchtop ) you must install washers or this plate to raise it up a bit to allow the quick change top plate tongues to drop in and out. You can use some 1\8" thick washers if you like, but this plate will distribute the pressure/loads much more evenly.
Note: You still need the flush mount quick change base plate. (Here)