Some FED original and type 2 or 3 , first series of model 4, Zorki 1, S, 2 and 2S and models 5 second series and 6 . eventually need a rangefinder adjusting in horizontal aligment. This adjusting is done by rotating the center core of the set. There exists a small prism inbuilt. It is the external auxiliary RF window. Drawing and pictures will clear explanations. Drawings from Novacon pictures from jay.fedka