I wonder if the National Newspaper Promotion Association (and I've spotted some of their ads too, in 'E&P' from that era -- so that might be a good source too) folded eventually into the Newspaper Association of America (NAA, now the News Media Alliance https://www.newsmediaalliance.org/about-us/). It might be worth contacting them. I do think that if you can't find a parent organization that's inherited the copyright, so to speak, you might be able to use the image as long as you mention the organization that originally provided it,.
Anyway, I hope that helps! Your work sounds interesting. Finally, since I do spend some time in that era, I'll also keep an eye out for similar ads/references to that organization, and let you know if I find anything. What's a good email for you?