Pre-Campy GS (1951) parallelogram rear derailleurs I found in a quick flip through the books:
THE DATA BOOK: 1947, chainstay hung. 1935, Nivex also chainstay mount
THE DANCING CHAIN p. 149 - J.I.C. Course was a parallelogram unit (1946-51), but not nearly as elegant as the Campy design. p. 150 - J.I.C. Tourism loks like the real mccoy, but chain-stay mounted. Same era.
So, by and large, it seems fair to conclude that the Campy GS, 1951, was the first successful parallogram rear derailleur from a firm big enough to market it.
And it was one heck of a lot better than the plunger units it competed against.
Still, someday I want to own and use the one that got away: the earlier Altenberger or later Suntour chainstay mounted units with horizontal parallelograms. I've only seen the A. once, on an "Altenberger Gruoppo" Cinelli at Cirque a few years ago. I think 1954, and fortunately too small for me.