Safety Clark Socket for 3/8 x 1 1/8 Side Rail Used for attaching accessories which have round rods sizes 5/8 ~ 11/16 (16 ~ 18 mm) onto 3/8 x 1 1/8 or 10 x 25 mm side rails, providing radial adjustment. Includes a thumb screw for tightening the Clark Socket to the side rail. Composed of stainless steel.