I got a reply from the person who sent the photo, he said, "...there were 20
boxes of books that I donated to a local library and there were some books
that went out of the house before I got involved. The guess of a book
divider appeals to me most...seems most likely to me. The previous owners,
now deceased, were University professors."
I wouldn't say this one is one hundred percent solved but I think this
answer is probably correct, thanks to everyone who helped identify this