We found this when we took down basement paneling in our 1968-era house. The black metal appears to be the actual flue with a cover on it. (The cover has words scratched into it, as if with a nail: "Gas only. Not for wood." But the woman we bought the house from said there was a wood stove there when she bought the house.) The silver-metal-looking stuff is actually very thick sort-of foil. So it looks like a flue surrounded by mortar then foil then more mortar. Also you can see in the last photo that the foil is folded over with mortar between the two folds.
We don't intend to use this but I have questions.
(1) Is this common? I don't have that much experience with chimneys, but I've never seen anything like it.
(2) Is it okay to have the furring strip that close, if we did decide to use it? If so, does the mortar make it okay?
(3) Because of all that mortar and foil, can I use plain old foam to fill that huge hole, or do I still need high-temp caulk?
Thanks a lot.