This allows a MakerGear slide notch mount to fit in the Drop In Paxtruder. It is a small U shape wedge out of 3/16 ths Acrylic. You have to peel the Covering off or it's too thick!Laser Cut - build the layers of the Drop In Paxtuder - add your nuts - leave the last layer off until you insert the MakerGear "Hot Head", first work the square closed end into the back of the extruder. Slip the retainer Plate over the "Hot Head" and slip it in the Joint. Then place the last sheet of the Paxtruder over the U shape wedge, I started it perpendicular and rotated it in place on the "wedge" 90 degrees to making the final plate on the Struder. Assemble the rest of the fasteners. The last bit it to take the extra fill plug for the U shape wedge and push it in. This is all about shear strength it has been tested and works for me. Your milage may very. I broke one during the first assembly. They are fragile laterally across the U shaped joint. It's small Print all 4 - have spares. Good Luck - Enjoy!