Looking for one of these older (ca 1960's-70's) focusers. Combination rack-and-pinion/helical - the outside of the draw-tube is threaded; the pinion drives against these threads as a "rack"; the draw-tube can also be rotated against the pinion for fine-adjust.
To be installed on the Austin Astro. Society 16-inch Ealing "Educator" Cassegrain (also 60's vintage) at Canyon of the Eagles Observatory. http://www.austinastro.org