I'm doing some historical data preservation work, trying to track down licensing for some really old (late 1970s and early 1980s) CP/M software. I'm using 'good enough to get into Debian main' as my ideal win condition here because it's a pretty high bar and if it's good enough for Debian it's good enough for pretty much everybody. Plus, I'm hoping to be able to produce a Debian package containing this stuff eventually for use in emulators. Back then people were really slack about licensing. Typically you'll see software contributed to a 'public domain' library with no explicit license but which contains a bare copyright statement. I have to write this off as if there's a copyright statement, the default license of all-rights-reserved applies. However, frequently there'll be software which doesn't contain a copyright statement at all.
I know that US copyright is weird; they didn't join the Berne Convention until 1988. This means that works published without a copyright notice automatically entered the public domain, all the way up to March 1st 1989 (provided they weren't subsequently registered for copyright).
This sounds like good news, but it may not be good enough for Debian --- countries other than the US joined the Berne Convention on different dates, so it's possible that a work could be PD in the US but still copyrighted elsewhere. And even then I'd still need some kind of paper trail to demonstrate that the work actually is PD.
So: from Debian's perspective, what's the degree of proof I need to provide in order to demonstrate DSFG-ness of works such as this?
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