These were hybrid units.
The outer part of the wheel was an aluminum alloy (once piece including the center hub)
that was riveted to a steel inner wheel. That's how all of the sabres were made starting in
1955 until the end of production. The riveting process changed, the dimensions of the inner
steel wheel also changed (starting in 1957) to accomodate larger brakes. Kelsey
Hayes made them for Cadillac with technology from Alcoa for the aluminum issues.
I have a few spares in my basement and 5 restored ones in my 1955 Eldorado.
BTW, they can be replated nicely by somebody who knows what they are doing.
Recommendation: Valley Wire Wheel in California. They are expensive ($1,000 per
wheel) but you will not regret it. I had mine done 3 years ago and not a sign of a
pit, flaking chrome or any other problem. Yes, they hold air well even with tubeless
tires as they should (they were designed for that).
Mike