Peter Jon White wrote, with his usual wisdom:
The NR derailleur was designed in the days of 4 tooth spreads between outer and inner chainrings, and works extremely well with 52-48 chainrings and the like. When riders started using 10 tooth spreads (42-52) this created a big problem for the rear derailleur. If the chain was short enough to not droop when on the small chainring, the upper pulley is pulled away from the freewheel cogs when the chain is on the large chainring, making for the notoriously poor shifting of the NR rear derailleur when on the big ring in front.
But the problem is solved by the spring loaded upper pivot, which (someone will correct me if I'm wrong) was first made by Simplex. The spring loaded upper pivot allows the derailleur to pivot forward when the chain is on the big ring, which then allows the upper pully to track closer to the freewheel cogs, making the rear shifting just as precise when in the large ring as in the small.
----------snip-------------- Now, way back when, in days of yesteryear, I'm sure that there was an earlier double-sprung Campy "mamser" (this is a yiddish word for the offspring of a less than formal union). If you look at pre-1961 (+/-) Campy rear drop-outs, there is a little hole above the hanger bolt hole. This was the stop for the top pivot spring of the infamous Campy Sport brass derailleur, which had a body very similar to the Gran Sport and Record, but only one jockey pulley. Another design that will amaze all who see it for sheer awkwardness. stampted to handle 16-22, but I doubt it. In any event, by driving out the appropriate pins you could in theory marry up the Sport top end with a record bottom and have a Genuine Dual Spring Campy. If, of course, you had a pre-62 drop-out (or drilled your own)...
You name it, there's always a harder way to do things...
Harvey "give me liberty and better components" sachs mcLean va