I'll check that out (removing the bolt to see what the hanger looks like). I'm hoping to take a ride today; I'd rather mess with it after the ride (I have a very specific window to ride in). But that bolt is supposed to be tight? I though the derailer had to pivot a bit on that, as it takes up chain slack. If not, then mine is way too loose (and may explain why my shifting isn't optimal: I have to overshoot to shift, then back off to avoid noise--and lately has been upshifting suddenly on its own when under full power).

How did you know that it was a 1984 part? Or did I post the date code instead of p/n? [Might explain why I can't find the date code, eh?]