This is a custom GT2 20tooth pulley made spcifically for 4mm Bore Nema17 Sanyo Denki steppers with Astrosyn dampers.
I made this because its impossible to find a 4mm bore 20tooth Gt2 pulley. I made the shaft longer to accomidate for the damper thickness. Uses 2x 3mm set screw and 2 3mm nuts. Even without the set screws its force fit.