I had a recent very similar problem with a completely rounded nut on a shock absorber.
I bought a set of sockets meant to grab rounded nuts but they didn't grip.
What did work fabulously well was my bostich spline socket. Find the smallest one that will just start to fit and hammer the crap of that sucker to get it wedged on. (You may need to use a fulcrum and lever with the tight spacing)
The nut was basically a cylinder, the groove (arrow) was added by pounding on the spline socket.
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