This is a connector to connect the Original SKADIS plate with this printed one (https://www.printables.com/model/282684-ikea-skadis/remixes)

You can also connect only the printed ones together with this connector.

The connector is 60mm from hole to hole so it easily fits into the holes vertically. The model is printed with a pause at the 82nd layer (with a 0.2 layer height) so you can put a nut in it. It is made for an M4 nut. After putting the nut in, you can keep printing. 

Thickness is exactly like the normal mounts that you got with the original SKADIS so you can also use it if you lost those.