I don't know much about Campagnolo from the 1970s, but if the second generation SR derailleurs were introduced at the fall 1977 trade show, it makes sense that they could have become available only in 1978! So both parties in this debate may be right!
Many items are introduced a few months before you actually can buy them. Of course, with subtle, cosmetic changes that don't require a lot of product engineering and testing, it also can be the other way around, that a running change occurs in production (say the molds for the old plates wear out and they decide to replace them with a smoother, simpler style, while making a few other changes at the same time).
Unfortunately, a date stamp may not be perfect evidence, either, because it signifies date of manufacture, if anything, not when it was available for sale. Date of manufacture of the upper pivot housing, to be precise. And I am sure that they made bunches of SR/NR upper housings at a time, and used them until they were gone. So I would not be surprised if some years don't even exist in the "date stamp chronology," because they had enough housings left over from the previous year.
A company for which I contract often sells products as "brand-new" that have spent years in their warehouses. Unlike car factories, which churn out most models all the time, smaller companies usually make a few products at a time, then change their assembly lines to make the next. The product is warehoused until it sells. When warehouse stocks run low, it is put back into production... So I would not be surprised if Campy made cranks for two weeks, then switched to NR rear derailleurs for 2 weeks, then changed to SR derailleurs for a week (few changes needed on the assembly lines), before revamping the lines to make front derailleurs... and so on. Maybe somebody who has visited the factory can illuminate.
Too bad Rebour wasn't working any longer, otherwise, I'd just check
Le Cycle and see what they say. Something as important as a cosmetic
change in a Campy rear derailleur surely would have caught his eye.
Jan Heine, Seattle
Vintage Bicycle Quarterly
c/o Il Vecchio Bicycles
140 Lakeside Ave, Ste. C
Seattle WA 98122