This is the disk styli #A-5000-7809.

These styli are ‘sections’ of highly spherical balls and are available in various diameters and thicknesses. Mounted on a threaded spigot, the discs are made from steel, ceramic or ruby. Full rotational adjustment and the ability to add a center stylus are features of the range. This makes them particularly flexible and easy to use. This can be used to probe undercuts and grooves within bores, which may be inaccessible to star styli.