Recalibrating Rangefinder on a Konica III?

Some time ago I acquired a very nice example of this camera but for the fact that the coupled rangefinder is well off, ie it reads 6 feet when true focused distance is 12 feet etc.
I am assuming that all one has to do is take the top plate off to get at the rangefinder settings. I can remove the rewind knob and the two screws under it on the top plate. I can remove the screw over the frame counter and the accessory shoe screw. However after this there seems to be nowhere obvious to go and the top cover remains firmly fixed.
Has anyone had any experience with this model that would be kind enough to pass on what I am missing?
Two pictures of the camera attached