@tripofmice Yes, and only publicly available books have an assigned ISBN.

In any case, I don't think there's going to be many people reading 70 year-old books. Keep in mind ISBNs are assigned to publications, not the books themselves. Books can (and often will) be republished, sometimes even verbatim.

If I were you, I'd first tacle the problem using ISBN and then you can figure out something else for older books.