This is a Manfrotto 2972 Double Ended Spigot has a 5/8" (16 mm) stud at one end and a 1/4"-20 threaded 5/8" (16 mm) stud at the other. It is ideal to link accessories to gripware such as the various versions of Super Clamp or any piece of gear with a 5/8" receiver.

Mounting 1/4"-20 Male (Connector 1)
5/8" / 16 mm Stud (Connector 2)