/r/whatisthisthing and/or /r/Whatisthis are the subs you're looking for.

Range finder. Probably for photography.

That's a rangefinder for old fashioned cameras. The one in your picture looks similar to this model from Saymon-Brown, but the "Meyer" stamp on it indicates that it was made by Hugo Meyer, another manufacturer of camera rangefinders. The designs were all the same, just manufactured in different places. "Occupied Japan" as your rangefinder says was much used for cheap optics at the time.

I think that particular model was used for the Speed Graphic camera, which was a very popular camera among press photographers.