Swivel Ball/Post Positioner with OR Rail Clamp. This is the Mount Post for Standard Universal Systems. Upper Post rotates 360° and can be angled at any point ± 90° from the vertical. Lower Post is 5/8″ (1.59cm) Diameter and 8.5″ (22cm) tall. Upper post is 1/2″ (1.27cm) Diameter and has 11.75″ (29.8cm) of reach. Lower Post includes built-in Rail Clamp.