Google Books shows the WD1771 available and in use in 1977 with more products using it in 1978.
(I find Google Books very useful for this sort of archaeology. Change the time range to "1977-1980", search for "wd1771", and then sort by date. Of course this isn't thorough, but it gives a good overview for little effort.)
Off topic but I wish Google allowed doing the same for web results, to search their index the exact way it was some number of years ago so that it would only show results that had actually been indexed at that point in time.
