Based on the Copyright Office database, the original copyright was by DC, with the renewals by Universal. It didn't look like the full run, but (a) a decent number and (b) it was a lazy search of just the full title. If the book changed names at all (even on paper), ran long enough that renewals wouldn't be required, etc., there might be more than the couple dozen renewals I saw. So, I wouldn't necessarily bet on it being clear, but depending on the run, some of it is possible.
However, even if the books are public domain, I recommend treading lightly. There's a huge difference between dealing with DC (who, by and large, seem to understand the public domain ecosystem and tend to turn a blind eye to piracy) and Universal (who will sue grandmothers on seeing their IP address download a torrent with the same name as one of their movies). If anybody finishes the research, finds some gaps, and makes them available, I'd recommend being absolutely clear (on any download pages, for example) on which issues are which.
