Vintage all the way!  No longer necessary since chain tensioners do the same thing (plus tension the chain) but were originally designed to prevent damage to the rear drop-out of an aluminum frame. If you have an aluminum frame and don't want to dent the rear drop-out when tightening down your rear wheel, throw these on your bike and tighten away! washers not included.  These are all that is left, when they are gone they will be gone forever!!