OK, to close the loop I redid the offending pivot by removing the upper pivot pin (rhs in the photo), drilling through for a 4mm BHCS, made a brass bushing to thread onto the screw, and secured the arm with a nyloc nut. Both studs spun freely on the body, Both are steel and are 6mm diameter. The pin was anchored in the aluminum body by some sort of steel insert. I would have ended up tearing the whole thing apart if I wanted to find out the details. As it turned out I had to put a step extension on the brass bushing to fit into a cylindrical recess created by pulling out the old pin. The bushing is threaded so it snugs the screw head to the body. Anyway, I'm confident of the strength of the new pivot pin and all slop is now removed. The derailleur shifts more crisply too.





And a view of the finished new assembly. The hex head of the cap screw is visible as is the nyloc nut on the other side. Yeah, its not original in appearance but it has served me for 40+ years, so worth a salvage.