If I remember right you can either use the metal lock ring or the black cap with gold emblem as a lock ring. I just used the black cap with the gold emblem instead of the metal one.
Rick is correct. If the nut is too tall for the plastic cap to sit flush on the handle, then you use the metal piece instead. I just replaced most of my handles with the power handles and it took me a bit to figure this out. I also just started cleaning and servicing my own reels since I can't afford to pay someone to do all of the ones that I have. I did 4 today. Only about 20 more to go before October gets here.