It's a small flanged tube of white nylon (at best) that's not made for bikes, but happens to be unimprovable for taking up the space in an allen-fitting fork crown or seat stay bridge when you have to put a nutted-style brake in there. We use these a lot. Onna counta our frames are made for allen brakes, and odd as it seems (to us, too), we can get our Dia-Compe centerpulls only in allen-front and nutted-rear. So the rear brake bolt is too long for an allen fitting. So, so, so you put this in, and it more or less converts the allen hole to a smaller nutted hole.