As I heard for years at work, when all fails, read the spec.
"Vintage Pair, Ambulance, Hearse Rear Qtr, Window Aluminum Cross Panels Cadillac
Vintage Pair, Ambulance, Hearse Rear Qtr, Window Aluminum Cross Panels Cadillac"
Based on that I'm voting sand cast aluminum. Very doable in the time, cheap and remeltable if the first casting didn't work.
I have in my collection of labor saving devices for the home a cast aluminum orange juicer that was sand cast at American Laundry Machine in Rochester back in the 20s. Aluminum left in the crucible got poured into molds that just happened to be sitting there, and the juicers got to ride home in lunch boxes.
The ductility of cast aluminum in that time left a lot to be desired, and many juicers came back to become remelt. Management knew, they considered it employee learning and research.