It’s a fairly universal HVAC component for the connection between the ductwork and the register grill/opening. Type it in a search engine and you’ll see.
Evening Dominic. Hope to find you well.
Not meaning to be offensive. You read the OP’s and my post?
OP: The boot or box below the register was steel, but I decided to stick my flashlight and camera in. The duct itself looks like some kind of rolled masonite or similar. The flakes on the transition between the box and the duct look like asbestos, but the coating on the inside of the duct itself doesn’t really look like the normal asbestos roll paper stuff I’m used to seeing, or the cement.
Questions: What is the ductwork for? What do you mean by boot? Does the HVAC ventilation pass through concrete?
The unknown material in side of the metal cylinder is fairly universal?
You think the Tom will post here again after all this commotion?
No one even help him create a narrative for his report.
Tom is not a newb. I think he is doing just fine, see below.
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Sorry - away for a few days. Oh, I haven’t been directly in this biz for too many years, but I’m keeping my clients happy. Every forum seems to have a life cycle. If you are into old tractors or old woodworking equipment, for instance, you’ll find yourself in a similar environment where a seemingly simple question can set people off. Either because it’s been asked and answered ad nauseam or because the appointed moderators found it lacking in some way. Fortunately, there is still a trove of good info available on here and even the prickly reactions usually have value of one sort or another. Can understand why people are wary of the mousetraps, though.
Anyhoo - thanks again for the clues, or absolute positive IDs, whichever they were. 
I was looking for my own info, rather than for a report, else I’d have put this in the emergency forum.
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