A meter exactly like this was in production by ICA, after the merge, the Zeiss Ikon brand was introduced with the same design. It is well finished and quite ergonomic. You see through the hole in the back side (below) and as you turn the metal ring around the degradée filter (above) makes the image fade into dark. There is no Exposure Index (ASA or DIN) on it, but the round paper (in the picture) that comes with, lists a set of films in production at that time. For those, the meter was "calibrated".



If you hold it with your right hand by the hanger it covers your eye avoiding parasite light coming in.

This is just a simulation I made with with image editing but it gives an idea on how it works. Imagine that you have this scene in front of you.

You must cover your eye with the Diaphot and look through the hole. Starting with the lighter part of the mask you get a bluish color.

As you turn the metal ring it starts getting darker.

Then you must decide when to stop. Like in any extinction meter the idea is to go the farthest you can provided you still see something.

That is somehow difficult when you start because you don't know if it is already dark or if you didn't look long enough. But with some experience it is indeed much more effective than wild guessing.