Lightweight (and low inertia) knob for easy manual filament loading/unloading.
Designed to tight fit onto NEMA17 shaft, but may be secured with set-screw M3.
Works best with Compact bowden extruder and related remixes.
UPD: Version for stepper bore 8mm added (for NEMA17 with planetary gearbox)
You may need M3 tap if you want to use set-screw (or just heated up screw :)
Also you may warm bore hole with soldering air gun before fit on stepper.