No, that is what you use to lock the cylinder in place when a round hole is drilled in soft material like wood of even plastic. The proper hole is actually supposed to be round on the ends and flat on the sides to keep the cylinder from turning (same shape as the cylinder). It's normally called a double "D". Generally a punch is used on steel panels to make that hole, in other applications a round hole is drilled and the tits on that piece bite into the softer material and secure the cylinder, The nut in question can be seen in my first picture, screwed half way down the cylinder. I don't have any of those pieces near me right now, but I think you get the idea.
EDIT: Just added this picture, the nut is close to the cam (lever looking piece)