There's clearly no mens rea on the part of record
shop owners at this
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.