[TEntwistle]

I really don't think that the "yellow" piece is Lexan or plastic. In Autodesk Inventor, when you are working on some pieces, you can make it so others are almost translucent so that you can see what you are working on better. (Or this happens automatically, I am just now learning Inventor, so I don't have it all down yet)

As for the upper bump in the center piece, if it is to connect to a support system, the bump gives the ability to place 2 screws or bolts in the piece to make it sturdier. Thus, it could have a use.