It seems the IPC cease and desist letter to the Guardian's director of digital publishing, Simon Waldman, over photos of Hitler in a 1938 Homes and Garden magazine was invalid. They did not own the copyright on the photos that Waldman posted on his weblog.

IPC "conceded that "after
extensive research ... there is no way of ascertaining where
copyright ownership lies after 65 years. Therefore, it is not in our
gift to either agree or withdraw use of these images and words."

For me, this prompted a mix of victory and fury. Yes, I could put
the scans back up on my site, but it was clear that they simply
hadn't made any detailed checks on copyright when I first
contacted them, and had hoped it would all go away with a
single stern email. Fortunately for me, in this internet age, such
clumsy tactics don't work. Their attempt to squash the problem
had simply amplified it."