Truth be told, in my first draft I made an educated guess at what this said based on the context. Over the weekend I found confirmation that I was correct. So I've reworked that annotation. Not that it's anything all the revelatory. But I love being able to nail down the meaning behind every entry and bringing a new Houdini fact into the world.
What did it say? You'll have to wait for the book to find out!
Oh what a tease.