User blog comment:Xilinoc/Ch. 571 (Yachiru's gonna wreck it)/@comment-11446176-20140305130913/@comment-11051098-20140306163609

The tsuba was "ordinary" at the time Kenpachi picked up the asauchi from the dead Shinigami. But ever since the moment Unohana first saw him and fought him, his Zanpakutô looked exactly like it does now, with the hilt wrapped in white bandages and the specific tsuba.

I might be wrong, but the way I see it, his Zanpakutô is in it's sealed state, he just doesn't know it. Or, it is constantly in its Shikai state, and he managed to obtain that as a child, before he met Unohana. But I see the first option more likely.