This Leitz microscope has a draw tube calibrated for 170–190mm optical tube length objectives.

Leitz microscopes standardized at 170mm optical tube length. Other manufacturers used 160mm.

The Objective Clutch allowed the objectives to be changed quickly, but stay centered. Note the two centering screws in the clutch mount.
The compensator (often a selenite sheet) was placed in this slider.
The large cutout in the body tube accommodated the Tube Analyzer.