Pretty sure the metal thingie is the top surface of the proper expansion plug already installed. Just remove it to cut the fork and the put it backk. Don't forget to take the height of the plug above the top of the steerer into account when you cut the fork. You should not use the star-fangled nut. Don't know why they included it unless it just came standard with the headset they supplied. If you wanted to unload that headset to someone with an aluminum steerer, you would want to have the star-fangled nut to go along with it.