An obscure novel from the thirteenth century has been attributed to Author X, although whether she was the sole author of the long work is often disputed. The year of completion has also been disputed but should be accepted as 1221, because in this year Author X wrote a diary entry about her joy at having completed the work. In this diary entry she also mentions a specific character who appears only in the last chapter of the book. While doubts may remain about certain aspects of the book's authorship, there is no doubt about its chronology.
Which of the following, if true, most strongly supports the argument above?