Not sure what it is exactly called. This is a small "plate" that mounts between the dropout and the derailleur body. It has a small hole in it where the end of the tension spring internal to the derailleur is inserted and a "tang" on it which if I am not mistaken, catches the dropout and allows the spring tensioner to do its job.
If I have totally confused you, then try this link.
I think you will see the part I am referring to. My Super LJ does not have it....I need one. Anyone got one to part with?