[Andy38]

531/2 is a nice camera.
To disassemble the lens, you'll have to remove the cover and the control knob of the rangefinder; then, you can unscrew the front lens; remember to replace it in the same position. Use a spanner wrench for the rear lens.
Perhaps you'll see the rangefinder needs adjustment but this work is not difficult.
Here is a site that explains that; it's in French, but, with the pics only, it's possible to see how to do :
http://www.suaudeau.eu/memo/rep/Moscou_4.html
Moscow 4 has the same rangefinder system.