I'm deep into the French stuff as you can see on my bike. But like John E, I had a Suntour Cyclone II RD on this bike earlier in it's life.....then a Suntour Superbe from my brother (who I traded the Cyclone with) after that. Also had A Nouvo Record RD before these Suntours and the bike did come with a Simplex SX610 The Suntours shifted the best with the Simplex coming in a close second because of it's similar slanted parallelogram design, then the Campy NR. The present mounted Mavic 810 RD/FD was a late aquisition I got in the 90's. I still have those other derailleurs that might perform better than the Mavic, but this Mavic 810 set just looks to be the most correct system to keep on the bike for good!
Chombi
84 Peugeot PSV