This is an extension ring for a follow/seamless focus systems. It mounts onto the lens focus ring and get secured with a toothed belt, which is most likely included in your follow focus system kit. The ring increases the diameter of the driven gear, simplifying focusing when using lenses with very long focal lengths or during macro photography.
You can download a parametric model for FreeCAD and modify it to suit your lens parameters. The attached STL file is made for an old Soviet ZM-5A-MC 500mm/f8, on which it is very difficult to focus on distant objects and infinity without a follow focus gearbox.