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Soviet camera manufacturers strove to keep their cameras as easy to service as possible, in conclave with the Soviet mentality of manufacturing for endurance. (All the more ironic that most Soviet cameras are so delicate.) One of the ways they achieved that aim was by making their rangefinders childishly easy to adjust. Horizontally, that is. Vertical misalignments are more, eh, permanent in nature.
This applies to all rangefinder Zorki's except the 5, 6, and 10 when KMZ got cocky and broke with Leica paradigm. It probably also applies to some of the earlier FEDs, although I haven't tested that because I don't own any.
I figured this out myself... heh!
Now exposed is an obvious hole in the front, through which you can see an inner setting screw. Work it with a small enough screwdriver while looking through the viewfinder to monitor your progress. Making sure the lens is still set to infinity, calibrate on some very distant object.
Jay Javier has a pretty good overview of rangefinder adjustment.