These are great US Military issue Gore-Tex bivy covers for sleeping bags. These are NOT the bivy cover for the modular sleep system often seen, this is an earlier version that was used with another sleep system. It is almost the same, but there are some differences- it has no zipper just buttons, it has a pocket for stowing socks or a sleeping cap, and the buttons are not aligned to fit the current issue bags.
That all said, these can be used with the current issue bags if you just slip it over, you can use it with a surplus intermediate cold bag, with civilian bags, or just slip your poncho liner in to make a lightweight waterproof bag. Lots of possibilities on these, and they are the cheapest Gore-Tex bivy you will find anywhere.
I got these as used, but if they were ever used it was very, very little, they look new to me. On the bottom laid flat they measure 7 feet long.