The repair involved recutting the threads on the swivel for a second time (having been done originally in 2008), which of course then required a correspondingly (further) undersized nut to be used during reassembly. It appears that instead of a 1/8" undersize nut being used, a 1/16" undersized one was fitted similar (or possibly even the same one) as that which had been removed.
How could the wrong size nut be threaded on? It wouldn't even start......