O.K. so first, thanks a lot for the helpful suggestions. It gave me something to work with for sure. What I did was to get a sewing needle and ready it into the very small hole (#4). At the same time I held the bench key in the hole (#3) where the stem would go. I kept pressing on the eye end of the needle expecting to hear or feel a click but didn’t. I kept up a firm pressure on the needle while trying to feel if there there was any kind of force loading onto the bench key. I then noticed that I had inserted the needle in rather far, about 1.5 cm. Then, as I was turning the bench key in the unwinding direction it suddenly felt easier and I could feel a kind of light clicking as I turned. I didn’t feel any force on it but it was definitely easier to turn than before. So, I turned it until I guessed that the spring was completely unwound.
I also tried holding the small tab in and turning the bench key to see if I could turn the hands but no go there, not even a budge. The good news though is that I think I can assume that the mainspring has been let down?
The balance still only moves a tiny bit with some rather forceful puffs from the blower.
I’ll continue now and disassemble and as I go I’ll try to see if I can figure out what’s causing it not to set .
Thanks again for your help!