Trying to find a good diagram of this

http://www.gobrick.com/BIA/technotes/t19a.htm

Was about as good as I could, but does not explain my problem.

Recently getting my chimney swept, I was given an estimate of 1k to replace
my disintegrated Smoke Shelf.

The guy was cool, said that is why I sometimes may get smoke in the room (I
do --- but only initially and it goes away in less than 20 seconds), and
unless I was selling the house, no big deal.

But, on his estimate, he noted that the chimney could be brought "into" code
with said Smoke Shelf.

Getting back to this Smoke Shelf, it is above the damper, and if it looks
like the diagram shown, how on Earth could it be disintegrated?

Color me curious...