Canon FD Vivitar Macro focusing teleconverter modified as a focusing helicoid.
Front and rear mounts were removed and replaced with T mount ring at back and M42 thread at front.
High quality 7 element teleconverter assembly (made by Kiron) is replaceable into helicoid body.
Lens cone is reversible to fit inside the helicoid backwards for focusing to infinity with lenses of approximately 80mm FL.

Rodenstock Modular Focus Helicoid is an expensive ready made alternative.