These go on the picatinny rails at the front of the crossbow.  For myself they best lined up on the 2nd notch of the picatinny from the front.  You can see in the picture they will deflect a little from the pressure of the bowstring which is why you should print them in TPU.  I have printed them in both 89A (yellow) and 64D (white) and both survival well but I'm currently using 89A.  I printed mine at 70% infill.  They use M4s.  The limb I'm using is 120lbs and when I tried a mix of PTEG-CF etc they broken after 1-2 shots, but these have held up for at least 30 shots now so figured I would share them.