Focus is via a fine-pitch screw mechanism controlled by this thumb wheel.
This Oberhauser-type mechanical stage consists of a top specimen plate manipulated in X and Y via two thumbscrews (A & B). Spring-loaded lever at "C" presses against the plate allowing the thumbscrews to move it in any direction. Unfortunately the spring (arrow) is broken.
Top view of the mechanical stage showing X & Y thumbscrews (A, B) as well as the projected position of the spring.
The Richebourg microscope also has the more traditional slide clips.