Jerry,
This is on my circa1970 Frejus - perhaps different in later years:
http://farm3.static.flickr.com/
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.
Good Luck!
BOB HANSON, ALBUQUERQUE, NEW MEXICO, USA
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