I wonder what the attachment holes are for too?  Could it be an attachment to make the Waki into a fighting spear?  I know HD has already stuck his Waki in a tree. 
That's EXACTLY what I thought.  But I can't quite picture them selling spear shafts.  Then I wondered about an attachment to connect two of them into a double sword.  No, I can't picture them selling that, either.  Maybe an attachment point for a basket hilt?  No, too long a handle for that to look right.
I'm stumped.