No offense, but who started this rumor?
All chassis were tech'd with tech marks put on the chassis (at IFMAR worlds). If there was a 'secret' car, wouldn't that be illegal for him to run it anyway? since it wasn't tech'd? You could tech only one chassis, and that's what you had to run. There's been LOT of pictures with Marc's racing car, with tech marks on the chassis. There was a 'picture' car that was little different from his actual racing car. May be Adrian M can clarify this since he tech'd them?
At this point, it's a 4th chassis Marc doesn't even know about?
Heck, I'd love to be proven wrong, but I think someone is just BS'ing on the internet.
