This is the cutter from the thread cutter attachment from Simon Hope. The cutter itself has a diameter of 16 mm. Made from hardened HSS. There is also a version available where the cutter has a diameter of 8 mm and there is an extra long version (90 mm) of the 16 mm cutter, the XL. All cutters have a 10 mm mounting.
A holder is also available for this cutter to mount it on the spindle of the lathe.