Inline Fabrication Ultramount Flush Mount Quick Change Base Spacer
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)