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. The ZAMAC logo made its return for 2017, after being absent on the 2016 releases (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).