That's a catchword, defined on Wikipedia as "a word placed at the foot of a handwritten or printed page that is meant to be bound along with other pages in a book. The word anticipates the first word of the following page. It was meant to help the bookbinder or printer make sure that the leaves were bound in the right order or that the pages were set up in the press in the right order."

Perfect. Thanks so much.

I always thought the term catchword referred to the decorative 'for/and/with' type words ala: http://welovetypography.com/tag/catchwords/

I wasn't aware of this use. Interesting!

Well, if you think about it, this practice serves a very practical purpose, if you understand the context.

Once upon a time, setting type meant placing individual, cast-metal letters in a composing stick, then manually justifying the line with spacing slugs, then transferring that line to a make-up form, and continuing the process. Which is hard enough, but the compositor had to work backwards while he or she was doing this, so it is not simple task. I worked for awhile at a print shop that still had foundry type and one guy who knew the art and craft. It was quite amazing.

So, you figure that the page numbers ought to be enough to keep things in order, except for the fact that, every now and then, you have front material, which also has to be numbered; plus, perhaps, plates of various kinds. So, page numbering was probably the LAST thing that got done. So, leaving little "placeholders" in the locked up forms probably helped the compositor keep track of where he was, and what was supposed to come next.