Bed leveling wheel/knob for adjusting the height of one of the four corners of the bed.
They include cavities for an M4 screw, as is the size of the bolts on my CR-20 pro 3D printer.

The edge of the wheels are designed in such a way that it is intuitive which way to turn the wheel to move the bed up or down. The edge consists of slopes. Turn the wheel in the "up-slope" direction, and the bed moves up (i.e., the nut is loosened), turn the wheel in the "down-slope" direction, and the bed moves down (i.e., the nut is tightened).