ZAMAC is an acronym for the Zinc, Aluminum, Magnesium, and Copper alloy that Hot Wheels and other die-cast toy cars are made from (cast).
A ZAMAC car is basically an unpainted car and/or base.
Kay-Bee Toys also had some 2 car sets (Same casting, one painted, one unpainted or ‘ZAMAC’).
In 2013, Walmart issued 18 unpainted mainlines for their exclusive 2013-Present Walmart Mainline ZAMAC series. These have a ZAMAC logo on the cards and are numbered 001 to 018. In all years but 2016, the Zamac Logo made an appearance on the blister.
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).