Philodemus' On Rhetoric was in Twenty Books

PHerc 1669, formerly identified with book 5, book 7 and book 10 of Philodemus' On Rhetoric, actually transmits book 20. The book number - a clear ? - is still legible in the final title of the papyrus. This means that On Rhetoric was in precisely or at least 20 books, depending on whether or not PHerc 1669 is to be considered the final book of the work.