I’ve got a simplex SGDG derailleur I can’t seem to throw away.
Either me or the previous bike owner buggered the spring tension feature on the spring thru-bolt. -pic below
So now I don’t know how to adjust spring tension on this thing. I gotta think this was/is a common foul up.
Is there an elegant repair that I have not been able to find on Google? Or scheme on my own? I bought an engineering degree a long time ago, and it’s not divining me a solution.

Side bar: this thing bolts on through the inside of the frame- so if you start trying to disassemble with the 5 mm hex on the outside, the anti rotation feature get rounded off.
like Click &Clack the Tappet Brothers said - French engineers never copy anyone. And NOBODY copies the French!!