My understanding (and we're obviously getting further from where I do understand things) is that studios play fast and loose with copyrights, because as I mentioned elsewhere, there's nothing illegal about claiming extra rights. Public domain means that we all own the copyright, so I can safely stake my claim to "War of the Worlds." But you can, too.
I admit that I don't know the Casablanca situation, but I know that there's an analogue in "It's a Wonderful Life." The movie fell into the public domain some years ago, but it's again considered out-of-bounds because the soundtrack has a separate, valid copyright. I wonder if something similar happened there, like perhaps the movie was distinctive and popular enough to make it impossible to create a copyright-free derivative. Or maybe it's a trademark issue. Again, I'm guessing by analogy.
The Moonstone situation may be more direct: Copyright statements are meaningless, today. They carry no legal weight, so you can honestly say whatever you want, and many companies simply have boilerplate--think of how many web pages you see that are perpetually copyright this year, even when nothing has changed.
Heh. I'd suggest returning to the Internet Archive with questions, since they host the (Green Hornet, I mean) shows and generally know what they're doing, but my last interaction with them was rather suggestive of them not wanting to deal with questions.
As to the original discussion of the script copyrights, the philosophy I'm using is that there are plenty of modern, copyrighted works built on public domain underpinnings. In every single case, the adaptation has a distinct copyright covering its original material, while the original remains in the public domain. That goes for the recent Broadway adaptation of "Lysistrata," Zorro novels, comics, and movies, and "Little Shop of Horrors."
I have to assume that, if I can sell copies of the earlier "Little Shop" without Greg Moranis and company suing me, then I can adapt the story (unless it's based on an earlier, copyrighted work, which well may be the case). If I can do that, then the same must apply to radio shows, even if there's a later-copyrighted script.