I went to MTO a few months ago and asked for a readout of all the cars I had ever owned.
The girl punched in my info and the machine started typing. I got 6 pages of say 50 cars. I have already located one of my L78 Novas. The girl was surprised on how many sheets were created. She asked me if I was a dealer
She told me I could also get a more detailed report on each car for $2.45.It would be worth effort for someone to get a job there and start digging.
If it is really important to you John you could hire a Private Detective. You could also talk to George or Frank at the GM Vintage Vehicle Services. They may not have any info but maybe an idea on where to look. You could also try any Cadillac forums. Any chance of finding out where the car was serviced?A long shot I know.
Good Luck