Cabrio 249 Stell cutting blade retaining Nut?

The steel nut that retains the circular steel cutting blade is unusual in shape:
I have a brand new Cabrio 249 Strimmer.
Tried to fit the metal blade, the nut didn't fit.
Have received exactly the same nut again from the agent.
This replacement nut is identical:
The hexagonal nut on one side has a circular hole on it, yet the other side the hole is Elliptical in shape.
This is not good engineering, assuming that the elliptical hole side of the nut is supposed to stretch when tightening the nut to hold the blade in position.
QUESTION:
Should the nut in question have a circular threaded hole on one side and an Elliptical threaded hole on the other ?
This is not a locking type of nut I am familiar with.