This is a quick release star knop for M8 threaded rods. I use it to fix my filament spools. So it is easy to change them.
How it works: The knob has a concentric and vertical M8 thread. Under an angle of 12° to the M8 axis is a 8.5mm bore-hole.
If you tilt the knob on the rod you can easily push the knob through the thread. When in position, bring the knob into a straight position. Then you can fix your part by turning the knob.
I needed to rework the thread with a therad cutter a bit to work fine.