Its the dust cover that fits over the focus and aperture rings on the lens body.
The balance ring looks like a filter but without any filter glass in it and is made from aluminum (I think, but could be brass under the coating). On the side of the balance ring after the diameter, it will have a number. Thats it weight. Different lenses take different weight rings (or none at all).
Think its odd yours was loose in the box. Mine came on the lens. As did the balance ring. However I ended up with 2 lens hoods and they look identical as far as I can tell.