So here is how this problem got fixed. After contacting Airstream (no help) Dometic ( no help either) the Italian sink and faucet provider ( all they could suggest was to buy a whole new sink and faucet) -- checking all the sources you all kindly suggested -- sadly with no good answers we resorted to old fashioned ingenuity.
We had bought a chrome plated plastic ring (for all of $2.50) sold for the "generic RV sink market" at our local RV repair place. It was about the right diameter but did not have metric threads so it only screwed on a little bit before it jammed up.
We found that by adding just a standard garden hose rubber gasket to this plastic ring to fill in the space so it grabs the goose neck o ring it tightened enough to properly seat the goose neck and hold it firmly in place so it functions just like a new one! One day it will surely fail again even though we will handle it with kid gloves -- but not to worry -- we already have a replacement ring bought from our local Camping World (for $2.50) ready to use when this happens!, not an elegant solution but who cares --- it looks OK and it works!
Thanks to all for your kind suggestions.