It's the thread that usually strips on these due to corrosion when you try to undo a seized damper. (heating first with a gas torch is the answer ).
I have repaired one by counter drilling the bottom and pressing in a brass, football inflator, adapter. One with serrated edge and then tapped to 6mm. Now better than the original.
Could you make up a brass adaptor that would press in and trap a standard steel washer against the bottom of the nut. Maybe worth a try ?. Brian