The lens mount on IIIG is a screw-in mount.
When the collapsible barrel lens is attached to the camera, pull out the lens barrel first and screw it in the lens mount, otherwise rangefinder doesn't work rightly.

Attaching the lens

Pull out the lens barrel, and then fix it.
Then, put the lens' center axis at the right angles to the lens mount.
After that, engage the lens screw with the lens mount screw, and turn the lens clockwise.