Writting "secret" letters or numbers through the use of a mirror was clearly used by Orffyreus. Here we see an example of how some kind of hook, when you apply a mirror, really is a "incomplete 5". Why?
You need to know "the Orffyrean Code" to understandt why it is placed just where it is. And what the real purpose is. It is really not that we found another 5, but it is about why? and why here? The placement is chosen with great accuracy, just as in the portrait!