That's a composite material of carbon Fiber/kevlar, it's what the entire shell of the F50 is made of. Stronger and lighter than steel, and perhaps aluminum. The entire F50 shell weighs only like 250 pounds...
Tamiya included decals for the rear wheel wells and some parts of the engine, as well as the interior. All the F50 pics didn't show the carbon fiber/kevlar for the engine but I put it on because I thought it looked cool..
http://www.twoguysracing.com/Catalog...0-%20124th.htm