Clint,

That's not a non-stock focuser on that 6-inch Deluxe Cave Astrola. It's a Carle helical focuser. You can see it in Cave accessory catalogs.

Helical focusers were very much in vogue in the 1970s. I had a Telescopics helical focuser on my 4-1/4 inch RFT.

-Lew