Using the Campagnolo timeline I thought the answer was 1982 where there is the first reference to a braze-on front derailleur -
'the Super Record front derailleur with black anodized arms, three holes and braze on fitting option'
But that's not to say Campagnolo didn't have it available before the publication of the 82 catalog. I would not disagree with Citoyen du monde's recollection.
You are more than welcome to disagree with my recall when we are speaking of more than half a life ago!
I did however find two catalogues from 1981 that show the braze-on:
I haven't yet found any 1980 catalogue showing it. This does not quite rule out 1980 as the launch date as cataloque photos were almost always taken months ahead of printing and therefore it might still be possible. In any event, I feel it is safe to say it was in either 1980 or 1981.