This slides into the rail on a Festool MFT to snug up the gap between the rail and the location pin on the front rail support.
It has a slight slope on the inside sides that match the angle of the location pin for a wedge fit and can be flipped or rotated for 4 new fitment possibilities due to possible wear.
I used 1/4-28x1/4 set screws as the thing is 6mm high. Drill your own larger holes to suit your needs.