The less plastic there is on any Simplex derailleur, the better, but only till after the 70's. The exceptions to this rule are derailleurs like the model SX610S from the early to mid 80's which is mechanically identical to the much sought after, hard to find and ultra expensive, SLJ6600 Aerodynamic. Only real difference between the two is the SX610S has plastic infills under chromed sheet metal parallelogram arms...and that difference discounts it over 100 bucks under the SLJ6600 (NOS), and as far as I know, they two models shift very well with good reliability. Simplex has other late 70's/80's derailleurs that have cheap and expensive versions that have the similar differences.

Chombi