Ian, that is what is known as a "dividing head" in the machinist trade. It works by a hand crank - mine is 40:1, so one full rev is 9deg; however, there are plates with holes that let you "divide" 360deg into smaller increments. A similar device, tied to the traverse mechanism of the mill table, will rotate the part as the table moves - which will produce a helical flute on a rifle barrel, for instance.