Prazisa Rangefinder

This little rangefinder, pictured here with its original instruction sheet and case, dates from around 1955. At that time, some cameras were focused by dialing in the distance to the subject but had no built-in rangefinder. Looking through this parallax rangefinder acheived this function.
Wikipedia explains: A rangefinder is a device that measures distance from the observer to a target, for the purposes of surveying, determining focus in photography, or accurately aiming a weapon. Some devices use active methods to measure (such as sonar, laser, or radar); others measure distance using trigonometry (stadiametric rangefinders and parallax, or coincidence rangefinders). These methodologies use a set of known information, usually distances or target sizes, to make the measurement, and have been in regular use since the 18th century.