I can see them now. I didn't have any problem putting in the floor panel with them in place. When I was doing final assembly, I must have pulled it out and put it back in a few times without any problem. The only place that I didn't final rivet was the right tip of the rear seat piano hinge. It gets riveted last when you are ready to close the floor permanently. I closed my floor just over a month ago so I remember it well .
PS. I was looking for the "C" shape but all I could see are the "L" shape stiffeners. I hope I am not seeing something else.