I think those derailleurs came out during Simplex's dying days in the late 80's
They were pretty cheaply made and some parts were even stamped steel when pretty much all manufacturers abandoned such materials for their derailleurs for good. I think most of the other parts are indeed plastic.....
I suspect Simplex made these as they tried to desperately catch just the bottom end of the market when the other makers left them behind in the dust with newer tech derailleurs with indexed systems.
Kind of a sad end to a company that was at one time, one of the biggest component makers in Europe.