Quote Originally Posted by Jerry Bodine View Post
At the bottom of this page you'll see some "Similar Threads" that contain some pics of the head. The top surface interfaces with the rail clamp of the camera. Both have the same diameter. The earlier models of the head had two simple pins sticking up and two screw heads (180* apart). The pins were spring-mounted so they could be pushed down to the head surface for whatever reason (was never obvious to me). These pins made it easy to quickly position the hole on the bottom of the rail clamp exactly over the attaching threads on the head's screw. The two screw heads sticking up matched the locations of notches on the rail clamp, so that when the rail clamp was tightened down the rail clamp could not rotate on the head. The large handle locks "pitch" position (aircraft term for nose-up / nose-down) of the camera by activating a disk-brake type of mechanism. This mechanism is a relatively large diameter joint that contains disks of a special non-metal material for friction control. The attachments for rail clamp-to-head and for head-to-tripod should be 3/8-16 threads to insure adequate strength.
Some variations out there... My gray Norma era rail clamp does not have 2 indentations for the pins to align with and with my black Norma tripod head, the 2 pins screw into head directly, not being spring-mounted.