I have read that the red hand, or rather hands painted with red color, have been used as a symbol of protests against the Vietnam War:
During the Vietnam era, it was a symbol calling for strikes against the war.
Unfortunately, the links provided in the article don't work anymore and the author, William Drenttel, passed away some years ago.
I am looking for sources for this statement, ideally photographs online, but also mentions in articles, books, etc.