This is the M2 spherical disk styli #A-5555-0300. It has a stainless steel base and stem and a ceramic disk.
|Thread mount (DG)||M2|
|Disk Diameter (DK)||20.0 mm|
|Disk Width (B)||2.5 mm|
|Disk to Base Width (L)||5.5 mm|
A-5555-0300 | M2 Ø20 mm ceramic disc | M2 | Renishaw