This is all great, but here's the annoying thing: it should be totally unnecessary. These are digitizations of public domain works, and there's no reasonable basis for granting them any copyright protection that would need to be divested with a CC0 mark in the first place. They are not creative transformative works, and in fact they are the opposite: attempts to capture the original as faithfully and accurately as possible, with no detectable changes in the transfer from one medium to another. It might take a lot of work, but sweat of the brow does not establish copyright, and allowing such images to be re-copyrighted (in some cases hundreds or even thousands of years after their original creation) would be pointless and disastrous.
Interesting. I never realized there was this much of a distinction between CC0 and the CC PD license, but it makes sense.