From the actual manual:
The condenser assembly, which can be used with other illumination systems, houses the heat absorbing glass and the lenses which focus the light rays to produce Beseler's exdusive "Cone-of-Light" illumination. Also incorporated in this assembly is the color filter cornpartment and its filter holder, designed to hold 6" color printing filters, variable contrast or neutral density filters.
The position of the negative stage relative to the condenser lenses is important when using the condenser lamphouse. The negative size indicator and negative stage lock can help you position and hold in place the setting to match your negative.
There should be an indicator on the right rear of the head, although it is missing on many machines (it's missing on mine.)
You can easily make one from thin metal, plastic, wood, even cardboard. Here are the indicator mark measurements for the different size negatives, all from the LOWER of the two screw holes:
4 x 5--one inch up
3 /14 x 4 1/4--2 3/16" up
2 1/4 x 3 1/4--3 1/4 " up
2 1/4 x 2 1/4--4 1/4" up
35mm--5 3/4" up.
Keep in mind that the precision required here is not very great. Actually, you can enlarge all the smaller sized negatives with the head set at 4 x 5, although that doesn't make greatest use of the enlarger's light intensity. If in doubt, just fudge a bit on the side of a bigger negative.