The original automobile racing wheels were indeed cast magnesium and were referred to as "Mag" wheels quite correctly. The cost, porosity, corrosion susceptibility and potential fire hazard soon had manufacturers change over to cast aluminum for road car use. However, the name "Mag wheels" remained even if technically not correct. A friend in graduate school in 1964 has a '64 Corvette fitted with Cragar aluminum wheels and both he and the manufacturer referred to them as mag wheels.