I may be missing the sandwich plate 40x40 that connects the side mount support to the base also, can't find it anywhere, checked all the paper in the packaging. I sorted it for now with a spare corner bracket (glad I ordered an extras, they also helped with the shorter 500mm seat supports.) I decided to mount the shorter seat supports 40mm lower, so interior to the base in the first channel with 8 corner brackets, at least for now, given my apart from the norm plans for mounting my seat. Taking pics along the way, and I will be starting my own build thread soon detailing my plans for mods to the P1-X, GS-5 seat with multi-channel tactile modded into the seat, etc..