As I recall, Simplex chose DuPont Delrin for their Prestige and Criterium derailleurs. The idea was that Delrin (previously used in automotive applications) was lighter than aluminum and (supposedly) made for smoother shifting. Problem was that the early all-Delrin derailleurs didn't break-in with use but rather broke down (IIRC according to "Two Wheel Travel Bicycle Camping and Touring"). Simplex solved the wear problem by adding steel inserts at friction points (which sort of negated the alleged weight savings). My entry-level Chiorda and nice Gitane Tour de France both had Simplex derailleurs and they both shifted well (the Chiorda's Prestige could handle a 14-28 freewheel in very hilly country). They were good derailleurs with a bad rep IMHO.
Steve Whitting
Prairieville, Louisiana USA
Website at http://ciocc-cat.angelfire.com/