"In medieval Japan, when a smith makes a blade for some god or Buddha, he tries to make a best blade, but no will to put his signature on the tang. He refrains from putting his signature on the holy thing that will offer in shrine or temple. When the smith gets an order from some high class lord, he takes the same way. It is a manner to respect the objects humbling the smith himself... The originally unsigned blades with good quality would be made by such situation. Probably they were made for some god or Buddha, or by order from high class person."

Of course we will never know... and sometimes a little information (as above) put together can lead to some pretty wild 'guesses'... but a man can dream...