Sorry to post and run! I think, unfortunately, that the hole is too wide for most of those two-screw socket plates. Just a hair over 3/8 of an inch on its own. I do have some spare thin brass lying around that could be used to patch the hole, I suppose...
In the meantime, I'll take the thing apart and check out what OAPOli suggested. If, after giving the lens a bit of a workout, I wind up really liking it, I'll think about buying a new shutter! Being the genius that I am, I bought a different 210mm lens at the same time, thinking I wouldn't win either auction. Hopefully one will turn out better than the other.