I'm probably that source since I think I am the only one that published what little information is easily available regarding the Nedor characters and the path of the company sales. The problem with tracking the sales is that Nedor, the company known by many names, published comics and pulp fiction, as did Fawcett who also went into paperback fiction. This complicates things as the various publishing ventures would be divied up, so when the company was sold, there wasn't necessarily a single buyer who bought the WHOLE company but just one publishing arm. The paperback division might go to one buyer, the magazines to another, or even further broken down. This makes tracking the copyrights problematical, especially where there is apparently no easily accessible public record of a transfer of copyrights as in the case of Nedor, Fawcett and Quality. The LoC can give us hints when we see who did the renewals although it doesn't mean they necessarily had the right to as in both DC and Claire Arnold renewed some of the later copyrights to the Quality material. It's doubtful that if the Quality sale included copyrights, that DC would have the rights to some titles but not others, especially when they did have the trademark rights!
The other complication in regards to this site and the cards is that a text book I recently read says that cases like Nedor where the copyrights are renewed but the current ownership is unsure or unknown is not the same as public domain and treating it as such can carry a fine. The text book does not say who initiates the action, the implication would be possibly the Library of Congress itself on behalf the unknown owners. Thus, the use of the characters, at least as far as they re-tell the copyrighted material such as their origins ala Dynamite is a violation of the US copyright law even though apparently companies are getting away with it.
On the flip side, the recent decisions regarding Sherlock Holmes, holds up that in regards to series characters, stories that are renewed cannot hold hotage the ones that are public domain or derivative works based on those public domain works. Thus, you may be able to use the Nedor characters as long as you don't actually reference the few copyrighted stories. The only wrinkle here is that it's their origin and first appearance stories that are still copyrighted, thus the general characteristics of the characters might be considered derivative of the copyrighted work. That the only things in the later stories that are free and clear might be the material that is unique to the public domain stories (the story itself, the villains, changes in the costumes such as the Meskin & Robinson take on the Fighting Yank). It's an area and permutation of the copyright law that I don't really know much about.