This is a timing pulley with 15 teeth, for a belt 6mm wide, type S3M (tooth pitch 3mm). I needed two of these for a 3D printer I was building, that had some pulleys missing from the kit (for the stepper motors). I had previously tried a design that uses OpenSCAD. It worked, however the overall diameter of the toothed bit was smaller than the aluminium pulleys used for the rest of the machine. 15 tooth pulleys this size seem to be more unusual, hence more expensive. This is an original design, not created using OpenSCAD. The one shown in the pictures (a bit rough, I know) is made from home-made ASA. I used a 0.25mm nozzle. The ones I made for my machine were made from nylon. The steel M3 nut should be pushed into the slot in the middle of the 5mm hole, then pulled into place (hexagonal recess) using an M3 screw. In use, I replaced the screw with a grub screw.