Yeah, I'm now proud and happy! (For a while?) I got the main spring back in place. Taking my time to make a plan paid off.
The main spring did not unwind totally because the shutter mechanism was inside the casing, so the spring only unwinded 2/3 turn and stopped against the casing wall. By making 3 simple tools from brass wire I was able to:
Hold the driving latch spring (00310-104) in the right position with my small U-shaped "tool".
Pushing the end of the main spring under the cocking lever arm with the most distant tool,
and catching it to drag it home with the tool in the middle.
Then I was able to put the screw that holds both springs and the lever that I had removed back in place!
But I have to admit: it is rather labourious to be an amateur!
If anybody ends up in the same situation as me and would want me to explain the above better, please let me know.
