These are 24 tooth pulleys that work with 5 mm pitch HTD timing belts. There are three bores available, including 1/2 in. Hex, 3/8 in. Hex, and Nub. Use the pulleys as they comes with a 9mm wide belt or add a Pulley Extension in between the halves to use it with a 15mm wide belt or two 9mm wide belts.
While assembling your robot, it can be helpful to add a drop of super glue to keep the pulley halves together.