Thanks for the quick response.
nesparky: 300.21 -- That's the one I remembered but couldn't find.
I've never noticed any kind of caulking, etc. in the photos here (and elsewhere) that show Romex through holes. (OK, many of the photos here are posted to show violations anyhow :eek
nor have I seen it in the few newly-built houses I have seen.
I did find the caulking online--one site had it for about $9 per tube.
Interestingly enough, I found some articles (via Google searches) that stated that fiberglass insulation, when the barrier material is covered, is better than wooden fireblocks.
lwinter31: Your explanation of "cats" sounds very plausible
As usual, you folks gave great information.