Follow focus gear for Zeiss Flectagon 20mm f4 (Zebra finish only).
The inside of the gear ring is designed to mesh with the machined grooves on lenses with the "Zebra" finish, as such it cannot slip. The gear has 110 teeth (modulus .8, film geared), roughly the minimum that can be fitted around the lens. This has the added bonus that the lens with gear fitted will still go in the original Zeiss lens box!
The advantage of this design is the ring cannot slip because it meshes with the castellations on the focus grip.
v3.1.4 : This is a good tight fit as printed from Shapeways. The print has 4 small tabs on the lens side which secure the gear if the print is slighly oversized, but these tend to break off if the print is a tight fit.