The “flat” Pulley fits on a 5mm axle of a Stepper Motor with a flat spot.
The “bearing” Pulley has 3mm “long” larger diameter on each side of the hole to push small bearings into it. This Pulley with the bearings can be mounted using a M5 screw. Use 2 pieces of the spacer to make movement easier for the bearings (Bearings: outer diameter 9mm, inner diameter 5mm, width 3mm).
The author remixed this model.
Made the inner diameter smaller and added a flat spot onto one remix.
Enlarged the inner diameter for the first 3mm on each side for berings to add on the other remix. The 3rd part (spacer) isn't really a remix :-)