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Part of the optical surveying instrument - Richtkreis 31.

Were used by machine-gun, mortar and artillery teams for topographic and binding definition of the angles (azimuth) of fire on the ground.

By tabulation and binding on the map is carried out shooting from the closed position, as well as shooting in low visibility conditions. 

Nice barn found condition, lens has small crack inside but the image clear visible, check the pics.