Here are more pictures to give you ideas. I'd get better ones but the cabinet is tucked behind two parlor cabinets which are surrounded by machine heads - I will be so glad when I move everything out (building is getting insulation put in ceiling today).
there are metal brackets on the long sides of the cutout, and a board underneath that rotates into them.
This shows the hardware - the insert has a small metal square that keeps it from being removable.
This sort of shows the metal support that the insert sits on in the Down position. It is a U shaped bracket with wood on top. I suppose that is one way to get it just the right height, as the base for the 128 is actually shorter than the base for the 99 - put in a different height wood lift and this would work for either.
the bracket is attached to the table -that other small piece of metal is the tab attached to the insert. In the picture they sort of look like they are one piece.
you can tell this spent most of it's time with the machine in place.
And with the machine in and the cover in place.
BTW- this particular table is made to use a specific knee bar that fits behind the skirt and into the hole in the base.