It's a Direct Connection spoiler made specifically for Dusters/Demons/Dart Sports. It is NOT a Camaro piece, and it is NOT for an Aspen or Volare. And while we're at it, I'll get ahead of the next one and say it's not for an AMC Javelin either. I know, I have looked into ALL of those spoilers and what it would take to fit them to a Duster. And a '71 Satellite, just in case anyone is curious. None of them are even that close, and they'd all take a ton of work to mount up.
There are several reproductions of the Direct Connection spoiler. If the spoiler is made from ABS plastic then it is in fact an original DC spoiler, and as noted already they go for stupid money even in not great condition. I would not say that spoiler is beyond use, I see that the pockets for mounting it are broken but that can be overcome if someone wants to spend some time on it.
I actually prefer the fiberglass reproductions, they're easier to deal with than the ABS although they're heavier, which can mess with the trunk springs. MAS made the best one as far as accuracy goes, but they've also been out of business for awhile now.