Originally Posted by
norwood
Originally Posted by
Retro Grouch 
That looks like a chainring nut to me. I've never seen anything that looks like that on a derailleur.
If a nut like that is in the derailleur hanger, could it be a helicoil or something similar? Some type of thread saver.
It's not a helicoil, it's a stock part. There are replacable derailleur hanger designs that are fastened to the frame with a chainring nut/bolt combination. My son-in-laws Bianchi's derailleur hanger is held on that way.
To the OP, yes you need both the bolt and the nut.