The overlays are soldered onto the bufferbeams on one side
and PCB soldered onto the other side then filed to fit.
Using PCB means that the bufferbeam insides are electrically isolated from the beams on the outside.
These gaps ensure that the sideframes can be joined to the ends of the bufferbeams without being electrically connected.
A fraction of the copper coating has been filed from the bottom of the PCB to ensure isolation during a later phase of construction.