So close! I bought a couple of 10-32 stainless nuts on the way home today (the bolt I have is also stainless). They should tap out neatly to 6mm, but while I have a 6 mm tap, I don't have the necessary 13/64 or 5 mm drill bit, so that's a project for another day.
But the outside of the nut, unfortunately, is a tiny bit too large. I'd already established that 10 mm is much too large, and 9 mm is too small. I now know that 9.53 is close to right, but still too big. I'd guess that the correct size would be 9.35-9.4. I guess I'l have to bite the bullet and file down one of the 10-32 nuts. I bet that one fairly gentle file stroke on each face will do the job. That's a big improvement over trying to file down a 10 mm, because the less material removed, the less the original regular hex shape is likely to get out of whack. Not sure when I'll get around to doing that, but I'll report back on it later.
Thanks again, bleukahuna.
JV