If you're running a Kossel (or any other printer where endstops need to be mounted on 2020 extrusion) and have endstop PCBs (like http://www.robotdigg.com/product/154/Microswitch-Board-w/-Lead-Wires-for-3D-Printers), this is for you. Put a screw with a T-nut thru the PCB and the hole in the mount; the peg goes thru one of the other PCB holes, and that's it. Needs support to print.