After extensive searching on the web, I have yet to find directions for adjusting the linearity of the rangefinder on the Leica IIIC. I can see what looks like an adjustment nut right above the rangefinder cam roller, but I don't know if this is the adjustment point. Is there a special wrench that is used to make this adjustment?
 
To be clear, I am not talking about the basic horizontal adjustment of the rangefinder. My problem is that when adjusted for infinity, the focus at three feet is off by about half a foot. I need to change the length of the rangefinder arm by adjusting the cam roller position.

The roller that fits up against the lens cam is mounted on an eccentric.  A tiny setscrew retains the eccentric in position.

Thanks for the information, Henry. I can see a small hole in the front of the arm that holds the roller, but I would have to remove the lens mount to access it. Is this really the correct procedure? This would mean removing and reattaching the lens mount to test each eccentric position adjustment.

I'm just working on memory here...and it has been a while since playing with a knob wind Leica.  The setscrew is more of a brake than a lock and there should be a method of turning the eccentric.  Is there flats between the roller and the lever?  Or a hex wrench place in the eccentric center?
 
I remember it taking several iterations on the Gokosha between infinity and the closest focus mark.  Going back and forth from the eccentric and the front adjustment screw.  But I don't remember having to remove the lens mount flange.
 
Henry

There seems to be a thin hex or octagonal nut between the roller and the cam arm. It looks like a thin wrench would just fit above the roller to turn this nut. I can probably grind a tool to fit, but I don't want to ruin anything. Does this sound like the right approach?

Hey, somewhere in my internet manual hunting I got a scanned Leica IIIF service manual.  Looks like my memory wasn't too bad.  No mention of a setscrew, but does show a hole.  Anyway, you want a copy?  It is 65mb so can't email it.  If you have AIM look for me as henrytaber2.  Don't leave off the 2.  I'll send it peer to peer...if you have fast internet access.
 
Henry

Better yet, use www.yousendit.com. Enter my email address ([email protected]/*  */) and upload the file. I will receive an email with a download link. The service is free and can handle gigabyte files.
 
Thanks for the documentation offer. I'll post what I learn about adjusting the IIIC rangefinder cam.

That's a new one on me.  Never heard of it.  But I registered and sent the file.  Have fun.

Thanks for the file, Henry. The manual reveals that a special spanner is used to adjust the thin octagonal nut above the cam roller. My next step is to grind a spanner. I'll let you know the dimensions when I am finished.