And can I just point out : and I quote:

Quote Originally Posted by Endless Twilight
Their purpose isn't stated anywhere, but selling them is the only thing you can do with them.

There is another weapon you can obtain from the Speed Square, and that weapon is the answer to the riddle.
So what is this then? And again I QUOTE:

Quote Originally Posted by Endless Twilight
Giving the Mythril to the weaponsmith will results in either the Great Gospel manual or a Gold Armlet. It has nothing to do with the Masamune Blades. As I've already said, they have no purpose other than to be sold.
That's a bit hypocritical isn't it?

Besides, The Flayer can be used as a weapon and not "just to be sold". So I believe you were misleading there.