Spacer for flush mount base plate. (Required if you are not routing the flush mount base plate into your bench.)

The flush mount base plate was designed to be routered into your bench to provide a nice flat benchtop.  However, If you don't want to route your bench top (i.e, mount it directly to the suface ) you must install this shim plate  to raise it up a touch to allow the quick change top plate tongues to drop in and out. 

Note: You still need the flush mount quick change base plate. (Here)