English trench periscope from the Great War

green / khaki painted bronze body with extensible head to fit the edge of the trench, hallmarked by the manufacturer and dated 1917; eyepiece equipped with a graduated scale for determining distances; carrying case in brown leather, carrying on the outside a screwable handle to support the instrument when in use; all components stamped with the British "Broad Arrow" military acceptance mark; in good condition with signs of use