They break and get worn out (and come loose), check what they look like on the bottom of your pallets every once in a while. Also they are hard to find, I doubt anyone except Makino sells them. They are kinda unique because of the radius on the underside of the "head". 

These are New Old Stock. A piece of paper in the bag has some more part numbers on it, see photo. Also is a photo of the bottom of a Makino Pallet, showing where these go (Pallet not included). I'm selling these for a lot less than Makino was asking 5 years ago, I have not checked price Recently.