By pure happenstance, last week I read a thread on OGR forum with a link to Atlas' parts site with a parts break-out for their California Zephyr train from a few years back. Clicked on the link and found the diagrams and a p/n for the dome. Called Atlas on Monday and my four O Scale Budd domes arrived today. Still can't believe these were available, and feel very lucky to have these...I've been looking for several years for ANY O scale dome by any mfr. or vendor, but preferably Budd domes.
These will be used in my four Clark Benson 21" dome cars produced in the 80's. A quick test fit and I'm confident these will work.