This is a DIY knob for the X-Axis belt adjuster on the Ender 3 S1 Pro. There is a 4 mm hole in the bottom for you to install an M4-0.7 threaded rod into it. The depth of the hole is 4mm. I bought a machine screw 45mm long from the hardware store and used a hack saw to cut it to the correct length (20mm works well and matches the original screw length).
The author marked this model as their own original creation.