In the pair Nib : Pen the first word names not merely a part but the part that is thrown away and put in fresh once it wears out. Which pair below stands in exactly the same relation?

A worn nib is taken out and a new one fitted, and a worn blade is taken out of a razor and a new one fitted, so this is the pair wanted. Handle : Bucket and Frame : Spectacles are both true part-and-whole pairs, which is why they tempt, but a handle is not renewed as a bucket ages, and it is the lenses rather than the frame that get replaced in spectacles. Wheel : Road is not even a part-and-whole pair, since a wheel is no part of a road.