An all-aluminium timing belt pulley for GT2 belt (2 mm pitch) 6 mm wide (up to 6.35 mm wide).
This pulley has 20 teeth and a 0.25" (6.35 mm) bore. This is the shaft diameter of a typical NEMA 23 motor.
It has two M3 grub screws (at 90° to each other), which require a 1.5 mm hex key (Allen key). The pulley is 17 mm long overall, has a 16 mm diameter hub, and a flange at the opposite end. Unlike rubbish aluminium pulleys often found on eBay, which have a flat flange than can chafe the belt, this is a well-made pulley, with an embossed flange, slightly rounded on the side facing the belt, and the hub is also chamfered.