Plastic 30-Series Draft Gearboxes, Draft Gearbox Lids and Spring Lids 10 pair per package Draft Gear Box, Draft Gear Box Lid and Spring Lid for 30-Series Couplers. Note: Uses #621 Torsion Centering Spring 2-56 screws are the most commonly used for center hole. Screw length will very depending on the mounting situation. WARNING: CHOKING HAZARD - Small Parts - Not for children under 14 years. Kadee? Quality Products Co. products are not toys and are not intended for children under 14 years old.