I stumbled into this thread again through a search engine. It's not what I need to find out but I see something that might be "off". I'm looking for how to access the inside part of the back plug.
I have a copy of the service manual and the aluminum wheel in the photo you posted should be up against the belt but the belt doesn't go around it.
According to the manual that is the Motor Belt Idler. It's job isn't to tension the belt but to keep it from flapping around. It says it should be adjusted so that the belt clips are in a permanent mesh with the wheel. If that is confusing message me and I will try to explain better.