Pogemahone is correct. The SLJ derailleurs used aluminum bodies and were very nice performing derailleurs. The matching retrofriction levers are arguably the best friction shift levers ever produced. Simplex used Delrin for roughly 20 years, starting in 1962. The aluminum body SLJ came out in the mid 1970s. By that point the amount of Delrin was pretty much indicative of the level in the line-up, with less Delrin equating to higher levels.

Simplex's reputation had unfortunately been tarnished by the boom era Delrin derailleurs. By the late 1970s, most of Simplex's former customers had switched to the better shifting and more reliable Japanese derailleurs. The sole, big name contract they manged to keep was Peugeot. The PY10 was the only common high end bicycle to use the SLJ, hence these derailleurs are harder to come by. The mid range derailleurs during this period often had aluminum pivot castings, with Delrin parallelogram arms reinforced with aluminum. The pivot housings were often anodized black and it is easy to assume they are Delrin. Simplex realized this and eventually went to the odd practice of having one housing un-anodized to show that it was not Delrin.