> This is on my circa1970 Frejus - perhaps different in later years:
> I suspect the "diving helmet" shaped braze-on is a Campy
> housing stop (#621). The housing is a stainless steel Campy
> (#617) with a frame end step-down measuring 5.85 mm. down
> to 3.85 mm. in diameter (which fits perfectly into this stop)
> ... and at the derailleur end the simple straight ferrule is just
> under 5 mm. diameter, for a perfect fit into the N/R derailleur.
> Once upon a time, Campy sold just the step-down ferrules
> alone (#620), but I imagine these are mighty hard to find now.
I've got loads of them as 5mm Shimano index outer cable comes with an assortment of ferrules. The stepped down ferrules are always left over. Should be the same for every bikeshop