I just saw that myself and came to see if anyone had posted about it! :)

Z-13... I'm horribly curious. I don't suppose the code is a clue at all to the identity, is it? Probably not, alas.

Someone who could have an appeal. So someone whose activities could be seen from a few different lights, not that that helps us a whole lot... but it has to be someone from Harry's past or who knows his family, I think, or they'd have to change an awful lot about the story.