The real bumper/ reinforcement bar that is behind the plastic bumpers. Magnet don't stick to it and it don't rust which both types of metal are pretty much like that.
i agree, the main bumper support under the plastic bumper skin, is aluminum.
Aluminum, stainless is too expensive and not ductile enough to be made into bumpers, especially of that complexity. As far as the magnetism, the magnet would only stick to low nickel stainless and its not likely that they would use 308 on a consumer vehicle.
