Removing the Backlash
After a fair amount of investigation, I discovered the issue causing the backlash in the rangefinder was the coupling of the rangefinder mirror mounting block to the lever underneath it that causes it to rotate. The block was not firmly fixed to the lever. Also, the block rotates in a hole in the rangefinder body and the grease used was thick and was causing some resistance. So the lever moved a little each time its direction was changed before the mirror mounting block would start to move. It should be understood that the amount the mirror rotates over the full range of focusing is very small so any slack in the system will cause a noticeable problem.
The rangefinder assembly can be removed from the front plate by removing the three fixing screws from the underside. Note: the three screws attaching the rangefinder to the front plate assembly are different lengths. It is important to fit them back into the correct holes or damage may occur. You may choose to desolder the wires to the PCB first or remove the PCB completely. The focus follower wheel will also need to be removed.
Underside of the rangefinder assembly. The blue arrow is the pillar the focus follower wheel is attached to. The red arrow is the screw holding the problematic lever to the bottom of the mirror block.
The mirror block and lever removed.
My solution to the problem was to first clean out the the thick grease around the base of the mirror block and to replace it with a lighter grease. I then used a spring washer under the screw head to better grip the lever. There is very little clearance below the screw once the rangefinder is mounted back on the front plate assembly so the thickness of the washer was quite critical.
Of course, once reassembled, the rangefinder was completely out of alignment and needed realigning. I found it impossible to get the horizontal alignment correct, maybe because the lever was in a slightly different position. I had to remove the rangefinder again and remove a washer from the adjustment screw so it had a greater range of movement.
It's worth mentioning here about the masks located on the front plate assembly, under the rangefinder, that the focus follower pillar has to pass through. This is how they need to be arranged before refitting the rangefinder. The focus follower pillar passes through the hole created.