The Mystery Cars were originally a Hot Wheels mail in offer in which the car received was left a mystery. In 2005, as part of the 10th Anniversary Treasure Hunt celebration, Mattel offered four mystery cars, each car came with a voucher. Collect all four vouchers and you would receive an exclusive 13th Treasure Hunt Volkswagen Drag Bus . The four vouchers completed a picture of the Drag Bus. 2005 Mail in Mystery Cars: * VW Bug * Baja Bug * Drag Truck * New Beetle Cup Each of these castings would be later found in the 10 Pack Kar Keepers with modified tampo.