Oh, Mercury poisoning has been known for a very, very long time, and certainly well documented since the late 1800s.

https://en.wikipedia.org/wiki/Mercury_poisoning

And yet my parents (both born in the 1960's) both played with beads of mercury on their desks in school on numerous occasions... It was pretty common

When I was at school (1980s) the teacher had us line up and put our fingers in a flask of mercury. It was a very peculiar sensation. I probably wouldn't want to repeat that "experiment" today.

Sure, I'm not saying the general knowledge of mercury poisoning was commonplace back then, but in the scientific community it was, I believe, well known already.

It was the Nazi soldier community they were worried about.

Mercury poisoning has been known for some time, however people in the general public knowing or caring about mercury poisoning is a pretty recent phenomenon.