Fine focus uses the "long arm" focusing mechanism enclosed within the support limb. The thumbscrew (B) pushes against a lever, which raises or lowers the objective.
Fine focus thumbscrew.
Fine focus thumbscrew side view.
Rack & pinion coarse focus. Knob (A) moves the trapezoidal support arm up or down. The arm extends through the support tube and out the bottom.
Extended support pillar is covered with a tube. The mirror is mounted to the tube via an articulated arm and half gimbal.