That's part of the original RD. I've never bought an extender. I replaced the derailleur hanger once, and it's the piece going from the dimpled doodad to the dropout.

I thought you were going to notice that my chain was incorrectly threaded from the top jockey wheel to the bottom one. I noticed it while looking at the picture last night. I could have sworn I did it correctly when I recently replaced the chain. My brain doesn't work so well, sometimes, so keep that in mind if you're tempted to follow my advice.