Thanks Renee! That is exactly the part I was searching for. Now that I know it's called a washer...
Trying to put an image here:
http://ecx.images-amazon.com/images/...5L._SS500_.jpg
If the image shows up, you can see (on the edge) that it's tapered. When installed between the derailleur and frame, this taper can correct an angled front derailleur.
