I believe that stop went on the downtube down by the bb shell . It allows you to run a piece of outer housing up to the front derailleur that has a stop built into it. If it was used up at the top the lettering would be upside down as stated earlier.
Campagnolo made a lot of odd bits, I have a clamp on shift lever that is a single lever,,, left side only, what would you do with that. I should photograph it eh.
peter weigle
Lyme, Ct.