The 1930-31 truck headlamps were plain steel painted black and a magnet should stick to them even after they are painted. I believe the factory built slab sided deluxe pickups used stainless parts but please double check this. The ring that held the lens in was stainless. The tail lights were plain steel painted black with a stainless ring to hold the lens in. The radiator shell was plain steel painted black. The 1930 radiator shell shape was continued into 1931, the only difference was the 1931 emblem was used in 1931. The cowl band was plain steel initially painted black but later changed to being painted body color (I think the date of the change is in the judging standards). I am not sure about the '28/'29 models but assume it was the same.
Charlie Stephens