ZAMAC is an acronym for the Zinc, Aluminum, Magnesium, and Copper alloy that Hot Wheels and other die-cast toy cars are made from (casted).
A ZAMAC car is basically an unpainted car and/or base.
The term ZAMAC car was first used in 1998. There were 25 carded versions available at the ’98 convention along with a baggy Zamac and a 4 ZAMAC car set.
Kay-Bee Toys also had some 2 car sets (Same casting, one painted, one unpainted or ‘ZAMAC’).
In 2004, Mattel released some of the First Editions in ZAMAC. These were a Toys 'R' Us Exclusive but were later found at Target stores as well.In 2013, Walmart issued 18 unpainted mainlines for their exclusive ZAMAC series. These have a ZAMAC logo on the cards and are numbered 001 to 018.
Walmart continued these releases in 2014 and 2015 (18 cars each year).
Walmart also issued a premium ZAMAC exclusive car for their Rewards program in 2013, 2014, and 2015.
The photo below shows a ZAMAC or Unpainted Metal Base (chassis).