ZAMAC is an acronym for the Zinc, Aluminum, Magnesium, and Copper alloy that die-cast cars are made from.
The term "ZAMAC" is also used for an unpainted car, and or base.
ZAMAC cars have been popular among collectors for some time. In 2004, almost half of the 100 First Editions were released with the ZAMAC look--clear coat over the bare metal. This was done intentionally. The first few were released in the vending machines at a 2004 convention. These could later be found exclusive to TRU in specially marked cases. Aqua stripe on top of case opposed to the usual magenta stripe.
The photo below shows a ZAMAC or Unpainted Metal Base (chassis)