Yes, they did advertise 5x4 Barracuda/Dart bolt size rims. I'm pretty sure they were 14x6 only.
I doubt they are magnesium. When aluminum gets oxidized from sitting out sometimes people confuse them with magnesium. Magnesium will look darker than aluminum.
They did make some rims with steel rim with aluminum center. I don't know if the 5x4 americans were like that. I doubt it. The two piece rims with aluminum centers and steel outers allways sell for less than a full cast aluminum rim.