You have to take out the bed to get at the nut. It is only four screws from memory but don't bend the rangefinder connector arm which is at the end of the bed on the viewfinder side. Your best bet would be to remove the retaining ring at the back of the shutter and remove the little U ring that holds the rangefinder connector arm to the bed. Once you do that you can take out the bed, shutter and all. Don't lose the little washers though.