The rangefinder adjustments suffered from difficulty to access them and their lack of stability. Initially I tried to stabilise them such that they could be accurately adjusted with the top cover removed and the cover replaced without them moving. This proved impossible but I'll mention here the modifications I made which made a big difference - just not 100%.
I found the adjustments, especially the vertical one, moved when the rangefinder cover was refitted. Tightening the two top screws and one of the screws holding the front bezel caused the movement.
The flash shoe has a spring under it that contacts the PCB mounted on top of the rangefinder. As the top cover is tightened down, this spring applies pressure to the top of the rangefinder. This seemed a bad idea to me so I removed the spring and replaced it with a soldered wire. However, this didn't make much improvement.
I found the rangefinder cover was contacting the main top cover before the two top screws were tight and further tightening was causing some distortion somewhere that was moving the rangefinder assembly. Similarly, the one bezel screw was causing the top cover to distort as it was tightened. A washer between the bezel and the top cover helped and fitting two washers under the pillars that the top screws fit into also helped.
Washer added under one of the top cover fixing pillars.
Washer under second top cover fixing pillar.
However, these additions were not a complete solution and I decided the real answer was to improve the access to the adjustments without the need to remove the top cover.
The horizontal adjustment is accessible (just) by removing the small slotted screw in the rear of the top cover. But the hole isn't perfectly aligned, there is a screw head that partially blocks the access and part of the rangefinder body also partially blocks the access. A very narrow screwdriver can be used but I found that inadequate for making fine adjustments.
The only reasonable solution was to remove part of the rangefinder body to make it possible to use a larger screwdriver. This is what I did.
The horizontal adjustment screw.
The rangefinder fixing screw and the part of the body obstructing the adjustment screw which was removed.
Before and after.
This gave a better access to the horizontal screw and was, I thought, as much as I could do. It certainly makes the horizontal alignment adjustable now.
Making the vertical adjustment accessible would need a hole cut in the top of the rangefinder cover. Normally, it's arranged such that the flash shoe is over the vertical adjustment screw and a hole is made under the cover plate of the flash shoe. In this case, the flash shoe doesn't perfectly align over the adjustment screw so it must have been decided to not provide an access hole. A mistake in my opinion. The position of the flash shoe was probably determined by the position of the rangefinder. Given the rangefinder had to be shoe horned into the camera (evidenced by the quirky offset rewind crank), there probably wasn't much that could be done and the flash shoe covers the point where the vertical adjustment screw is below but not in a way an adjustment hole could be provided. However, I decided to make one anyway. It's not directly over the screw but by slotting the hole it has been possible to make the adjustment screw accessible.
The Seagull 205, from which the rangefinder is taken, seems to have had a similar problem but, in that case, an access hole was provided, but it also had to be slotted.
Seagull 205 vertical adjustment screw access hole.
Of the original modifications I did, I removed the washer I fitted behind the bezel as it didn't make a big difference but I did leave in place the two washers under the rangefinder cover pillars. I think this removed some stresses on the rangefinder and will, hopefully, make the adjustments more stable. But the main improvement is being able to adjust the rangefinder without removing the cover. Since doing that, I've had no further problems.
Postscript: To create this site, I had to dismantle the camera again to take some more pictures. When I reassembled it, the rangefinder was correct and needed no adjustments. A definite improvement.