If you look down a little further, you can see that a valid copyright notice was actually required until 1989, though from 1978 on you could file an amendment of sorts. And yes, this explains a lot of the poorly-packaged movies you find in the dollar bins around the country.
As for comics, yeah, I can confirm that Charlton had some terribly-formed notices throughout most of the 1960s. Oddly, they did a lot right earlier on, and worked out the bugs around 1967-1968, so I can't imagine which legal eagle decided to "help," there. I'm sure that other companies also did this incorrectly (after all, you couldn't walk two steps without tripping over an independent publisher, back in the '80s, and not many of them had any business sense), but I'm not actually aware of any. Someone with a decent collection of indie books (and the underground books for the decade or so before that) might find it interesting to check, though.
Oh, and the Gutenberg materials are almost always based on renewals--I think the administrators find it easier than worrying about the original copyright and whether it was corrected during the limited window. Usually (especially for the science fiction works), there's a note at the beginning or end of the text that explains where the story was found and that the copyright on that work wasn't renewed. For example, Nourse's "Image of the Gods" has the following: "This etext was produced from The Counterfeit Man More Science Fiction Stories by Alan E. Nourse published in 1963. Extensive research did not uncover any evidence that the U.S. copyright on this publication was renewed."
And I have to back up Eric. Piper's work is fantastic, especially if you enjoy comic books, and it's a real shame that he ended his life.