[CR]Please identify this mystery part:

Example: Books

From: "devotion finesse" <devotion_finesse@hotmail.com>
To: classicrendezvous@bikelist.org
Date: Sun, 21 May 2006 21:28:51 -0400
Subject: [CR]Please identify this mystery part:

Hello. Please pardon my ignorance, but would somebody please explain the function of the part attached to the rear brake mount bolt seen in the picture linked below? http://www.theracingbicycle.com/images/Legnano_Roma_Seat_cluster.JPG I have never this on another bicycle before. Matthew Bowne Brooklyn, New York