In the film Monterey Pop, a poster in the background says "Hiroshima" and has a mushroom cloud. What's it for?

I am not entirely sure when it is as the film doesn't have any notable scenes to go by around the time, but a group of people are sitting against a wall, and on that wall is a poster with a mushroom cloud and the word "Hiroshima." I am trying to figure out what the poster might be for, and I wondered if it might be for the film "Hiroshima mon amour." However, as Monterey is in California and it is about a decade after "Hiroshima mon amour" was made, it seems unlikely. Any ideas?