Due to the fact that the metal rod through the center of traditional mooring buoys can be subject to hidden corrosion, prudence dictates that they should be inspected regularly, and replaced as necessary.
It's always good to have a spare on hand.
This product is a replacement for the swivel found at the lower end of the mooring iron. It is 4" long and fits all sizes of mooring irons; see Accessory Products, below.