An Ikonta has been described before on this blog. The Ikonta portrayed now is the later version with a built in rangefinder, i.e. a way to measure the distance to a subject, hence the name Mess-Ikonta (Mess is german for Measure).
The Mess-Ikonta is firstly a normal Ikonta similar to the one already mentioned. To that successful design was added a mirror system and a distance knob. The images speak for themselves:
The Mess-Ikonta 524/16 (serial# U66625) with its easily recognizable rangefinder (the two small squares to the right and left of the main viewfinder).
Seen from above the right distance knob give away this Mess-Ikonta.
The Mess-Ikonta seen from behind. The rangefinder is the little lens on the left of the viewfinder.
To use the rangefinder look through the left window. You will find a direct view of the scene in front on which is superimposed a much weaker image. Look at the part of the scene you want to be in focus. Rotate the right knob until the images precisely overlap at that part. Next take the camera down and make a reading on the scale.
The use this reading to set the focus distance and look through the main viewfinder, compose the scene and trigger the shutter.
The rangefinder knob and scale. The little round hole that shows a red (or white) tab is part of the double exposure prevention mechanism, indicating whether this frame has been shot or not.
As can be derived from the above picture the rangefinder is of most use at short distances. This is also where a precise setting of the focus ring on the lens is most important. Specially at full aperture. Beyond some 4 or 5 metres it gets pretty inaccurate.
So what is the deal with the rangefinder? In practice I found it actually less useful than I initially imagined. At person-to-person distances I can usually guess the distance quite well, and at larger distances the DOF increases anyway. Unless shooting full open at f/4.5 one can usually get away with a much a more coarse distance guess (and setting) and stop down accordingly.