Really Right Stuff ball heads now utilize the patented R-Mount interface for mating the clamp to the ball head's stem in the most secure manner possible. The R-Mount's rosette will prevent twisting at the coupling point between the two pieces of hardware over time, and will assist in eliminating "play" that results from regular use on the earlier tongue-and-groove designs. Users who wish to use an existing legacy ball head with an updated clamp will require a R-Mount CL Adapter. The adapter is a simple interface which will be installed (via the supplied mounting screw) between the ball-stem and the clamp, with the R-Mount rosette facing upward, and the tongue-groove key facing downward. Specifications: Country of Origin: USADiameter: 0.79"Height: 0.193" Really Right Stuff R-Mount Clamp Rosette Adapter RRS-9000002 in Black