The only copyright date in the Dover reprints is that of the original book, not the reprint. Which was 42 and 43, and copyright Crockett Johnson. Which, unless the book was pre-1971 or post-1990 or so, and assuming it was a legitimate reprint, should put it as PD in both the original and the Dover Edition. (When did the law eliminating a correct copyright statement go into effect?)
With that said, I would be happy to spend the $30 or so to get the original HC and use that as the source material if it would be viewed as safer by the copyright gurus on the site.
I envy you the Del Ray editions; those are not easy to find at all.