Photo for the rule booklet. I've only seen about a half dozen sets in my lifetime, so this is a first for me. It's an obvious printing boo-boo. Are a lot of these errors floating about? Was there an effort to recall? Was it the very last print run and the powers-what-be decided 'Who gives a smeg? Let it go."?

Inquiring minds...

I'm reviving an old thread, sorry.

deimos probably hit it on the head. Just an error in bindery.

If these books were printed on a sheet-fed offset press, that particular spread (or signature) might have gone through the press at a skewed or shifted angle. Sometimes a sheet doesn't get picked up correctly and enters the press skewed. The press operator didn't notice it (or didn't care). It made its way to the bindery, where again, it went unnoticed, and was bound into the book you show here.