The author references, at one point in the page, the fact Golden Age comics characters may NOT be in Public Domain, and that their free use by many publishers may simply be permitted to avoid problems with the biggest guys (like Marvel, or DC, or others) and for problems proving the ownership of the characters. Is this just an assumption, or there are any sources behind this affirmation? Also asking because, if I recall, at the time publishers and copyrights owners were forced to renovate copyrights every here and then, to avoid its expiration, and I'm not sure the copyrights retroactive measure which came later works in those cases too. I don't have any complains about it of course, just a curiosity.