Named for Sheldon Brown,
who bugged QBP to have them made, and thank goodness for that. The deal is: Modern frames have recessed allen fittings for the brakes, and they make it harder to mount fenders. They force you to mount the front fender in front of the crown, which is neither the end of the world, nor as desirable as mounting it in back of the crown--for easier on and off.
These nuts sub for the normal allen brake nuts, and do double duty as fender nuts. The 13mm one is the front one; the 10'er is the rear. Stainless steel. Just the fender nuts, you supply bolts.