I found them when I was searching for commentaries about the chocolate cigars - and Snoopy.
(As in, 60 years ago and more, pretty much NO one knew that chocolate was dangerous to dogs. It wasn't just Schulz; even people who should, in theory, have known better - such as the author of the Encyclopedia Brown series - didn't know. My guess is that back then, Americans were more thrifty in general, so they wouldn't have dreamed of wasting candy on pets, and so vets had relatively little evidence of what was bad for dogs and didn't "need" to make constant warnings to the public.)