To get to that spring and flip if back over the keeper one needs to take apart the front. The first thing the repair manual asks you to do is to remove and trash the indicator plate for light and dark scenes on the top of the shutter housing. Later it indicates you should peel the backing off a new plate and put it in place. Well, obviously a new plate is not available , so after the screws are out, you can slowly work the two pieces apart with gentle pressure. No up and down bending as this will bend the indicator plate.
There are 3 screws that hold the two metal pieces of the shutter housing together. To get to them I extended the bellows and used a screwdriver like this to come in from the back: