Quote Originally Posted by chaos View Post
I have a (hopefully) fairly simple question. On the Part 2 and 3 cars, there's a greyish metal box behind the Mr Fusion prop. I'm wondering what this is. Also it doesn't seem to be present on the Part 1 version of the car and I always thought the only change to the back was the plutonium chamber for Mr Fusion. Are you aware of any other difference between the nuclear and fusion versions?
The grey box you see forward of the nuclear reactor is an ammo box covered with a large heatsink. This was added between Part I and Part II, presumably to fill the void from the correct parts that were supposed to be there but were lost. Originally there were two round metal cans but those were removed to install the lightning rod "Trolley Pickup" base. After removal of the lightning rod base it the round cans never made it back on the car.



The basic revisions you see to the car are:

Part I
Mark I - As you first see it backing out of the van in the mall parking lot, Two round metal cans are directly forward of the nuclear reactor.
Mark II - Modified with lightning rod "trolley pickup". The round cans are removed and replaced with the lightning rod base.
Mark III - Mr. Fusion added, lightning rod base removed in front of reactor

Part II
Mark IV - Ammo box with heatsink added in front of Mr. Fusion

Part III
Mark V - Whitewall tires, hood box time circuits
Mark VI - Train wheels

Also keep in mind that there were also variations between the A, B, and C cars, offroad stunt cars, the Oxnard wheelie car, and the fiberglass mockup. Most people try to build an accurate replica mostly matching the A or B car with a few of the interior details accurate to the C car.