The 10-32 set-screw will tighten down on the shaft to provide a snug hold and keep the pulley from rotating independently of the shaft it is mounted to.
The 0.4” inside width provides space for a ⅜” belt-width and the set-screw nests down inside the teeth so that it does not interfere with the belt.
The pulley flanges are crimped on and will keep your belt from walking if your pulleys are not aligned properly
This pinion pulley use a 3/32” Hex Key (sold separately)