Harold- here are three photos of the black C&O from the top and bottom. Two photos of the sienna AT&SF, also showing the simulated coal load.
Then this is significant, I just realized. The C&O weighs 2.1 lbs, and the AT&SF weighs 1.12 lbs. Both are non-magnetic. The third photo of the C&O shows the frame, where I sanded away more of the paint. So, might I conclude AT&SF is aluminum, the C&O is pot metal (not brass)?