Yeah, the fact that nobody challenged Dynamite about it, means it's pretty likely that neither DC or anyone else can actually prove ownership of those rights. It seems likely that the rights were never explicitly transferred.
It brings up a whole category of public domain works that PDSH wiki hasn't even touched on yet...orphaned works. That is, works that technically were renewed or created after 1964 with valid copyright...but may not actually be owned by anyone because the orginal entity that created them disappeared before rights were formally transfered to anyone. For example, I think the Mego and morethe obscure Ideal toy lines may fall under this category, but it's not clear.