Look at the specs on the NPT S40 3/4" nipples.
http://www.msi-products.com/Specs/Br...ples-Specs.pdf
Sounds very close to your mystery pipes..
Lift up that fiberglass and stick your finger in the hole and see if you can
feel any threads.. If it's a nipple, there will be a few showing..
Anyways, I wonder if you could turn-down 1/2" nipples (using a lathe)
so they would fit inside your pipes, for soldering??