This is Ica Diaphot extinction lightmeter. Here is it's sequence of operation (please correct me if I got something wrong).

1. Look thru the hole in the meter
2. Rotate the outer ring of the meter so that the image seen thru the hole gets gradually darker
3. Stop just when it gets dark enough so that nothing can be seen.
4. Read the correct shutter speed and aperture values from the meter

When using this meter I got surprisingly accurate results, but only in the sunny 16 conditions. If looking for an accurate meter of the same age, I recommend getting one that uses POP (printout-paper" for measuring light intensity.
Pop can still be bought from a couple of sources in the internet. 


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