Author here (and original poster). That's a great idea. I'm going to write them to see what they think about the origin of the symbol, and will update the post if they respond.

I've always loved the location of that museum by the way - not only beautifully situated facing the water, but also across from a park frequented by sellers of hashish (at least it was when I lived in Lisbon back in 2012). The big iron fence dividing the licit drug museum from the people selling illicit drugs right in front of it has since become my favorite metaphor for the history of drugs.

You can try to ask it to this professor, he will probably know the answer or will be interested and find it. His email is on the web page:

http://www.museus.ulisboa.pt/en/jpsdias

Ahah indeed. Briliant. passing through that street will never be the same to me again.

There is a recent reddit thread about late 60s usa anti drug efforts, with some history about the status of drugs in the decades before it. Interesting.