How many authors are there from the 20th century who remain anonymous?

What authors from the 20th century have never been identified?

For example, someone who wrote either fiction or nonfiction whose identity–real name, real photo, real public appearance–has never been confirmed?

Are there any well-regarded, or famous, or positively received books from the 20th century whose ownership has never been conclusively established?

by the_eiger_sanction