To clarify, in case anybody wants to pursue this...
Comic strips aren't periodicals. Researching the copyrights requires going in for each strip (yes, one entry for, like, three panels), sorted by owner rather than title (maybe the publisher, writer, artists, or some random guy), and not always labeled clearly in the renewals.
The ownership issue was also troublesome, because the scans of the strips weren't clear enough to read, making it impossible for us to guess what to look up.
Lastly, most strips eventually ended up in the hands of the syndicate that distributed them to papers, and whereas comic book companies were generally fly-by-night enough to not care about or understand (depending on the company) copyrights, national companies that dealt with newspapers generally did. There are exceptions to this, but the way to bet is that comic strips got things right, from the few checks I've made, over the years.
If we eventually end up with waaay too much free time on our hands for research and can get clearer shots of those copyright statements, we might try to clear them, and (as mentioned above) anybody wanting to dig in would be appreciated. But as it stands, it's "needle in a haystack" territory, with too many unknowns and no evidence or memory that the copyrights were ever checked. And since not many people come here for the comic strips, it doesn't seem like a troublesome decision.