Since I mostly lurk in this forum, I thought that I would try to contribute a bit. So here is a picture of my tool in the box. This one carries the "Helios" brand but was made by Schneider & Kern in West Germany late in 1969. There is a shorter and longer protractor beam which can be inserted.
Here is a close up of the head. The black lever clamps the long arm or short arm in place. These are keyed. The lens is above the lever and has an adjustable focus. The black object to the right of that is a lens cap which I have removed. The knurled knob to the top left is a screw which is connected to a worm gear. When engaged (ein) it turns the gauge by small amount. When disengaged (aus) then the gauge can be turned by hand through large angles.
Here is another view. The knurled ring visible on the side is used to lock/unlock the angular motion. Loosen this before engaging/disengaging the worm, and use it to snug or lock the protractor in place.
Here is a very blurry shot of what one sees through the eyepiece. During a full revolution the angles run 0 ->90->0->90->0. Each degree is broken up into 3 groups of 4 divisions each, so 12 divisions per degree (each being 5 minutes of arc).
Here are the instructions in five languages (German, English, French, Spanish and Italian). The accuracy of the dividing circle is +- 5 seconds of arc and the overall accuracy is +-30 seconds of arc. The test table shows the error in angle in arcseconds.
Here are the other two languages and the cover page:
Overall a very nice tool, and one that will still be working in 100 years.
Cheers,
Bruce