What is it?
This is a spacer that goes between the guide block on the x-axis rail and the bracket mount for the heated build platform for the Cetus MK3.
Why is it?
It's meant to compensate the dent in the fold, that you can see in the photos. If the one screw that holds the bracket in place was tightened, it would tilt up the opposite side of the bracket. I'm not sure, if I just received a faulty bracket, but it solves my problem and maybe also for others.