There's clearly no mens rea on the part of record shop owners at this
point.
Generally (by my understanding) mens rea means you knew what you were doing, it doesn't require you to know that it was illegal.
There are different levels of mens rea, but such a case would likely fail on all such levels. I don't think you can prove that a record shop owner even knew that he was in possession of that album. As for knowledge of the contents of the album, again, I don't think you can prove that a shop owner knows the contents of all his albums. And on top of that, I'm not a legal expert, but I would think there would be some leeway given for the grey-area nature of this particular album.
What shopkeeper doesn't know what stock he has? And we're not talking about the contents, we're talking about the cover, it's hard to miss what's on the cover of an album.