I'm pretty new to Japanese swords, but very familiar with European ones. The European swords also have a lot of fake signatures, but even within the same workshop of the original smith, those signatures often vary. Because of that, those blades are generally judged by style and workmanship rather than by the signature itself. I would think a Japanese smith could far more easily fake an exact signature than make a blade of the same style and workmanship, as that is where the real skill lies. And I seriously doubt that a hand chiseled signature would be exactly the same from one sword to the next, even if it is the same smith. So wouldn't a blade need to be judged by its style and workmanship rather than just a signature?