Gosh I'm scraping the bottom of the barrel of my memory and knowledge but in essence I don't think there is a difference. One tells you the distance to set the lens at for correct focus and the rangefinder worked on the same principle of bringing two images into one the distance being read off a scale and then entered into the necessary equations with all the other parameters to give bearing and elevation to the guns. The optics were set feet apart to give good trig readings rather than inches so merely a matter of scale. Range finders are probably lasers now as in modern Leica range finding binoculars.
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