Hello Savatore,
I've never heard of a Byakkotai tantō. I mean, I know who the Byakkotai is, but I've never heard that they made any tantō, and I would be very surprised if there were such a thing. The fittings don't teach us anything. You would need to send the tantō to Japan for authentication to know who/when it was made.
Or, you can try sending detailed photos of the blade (not the fittings) to a site called Nihonto Message Board to see if the guys there can provide any more information. It's quite hard to date a tantō like this one, especially if it is out of polish.