OK. I went onto the CSI site and called their staff, explaining what I was doing. They said my future in-law will need to provide his link (birth cert) and his father's (death cert), along with a filled-out request form for any information they can release.
The state has a 50 and 100 year limitation: After 50 years, some information like circumstances (date brought in, conditions of what happened). After 100 years, names of biological parents.
I told them I was informed about a newspaper article announcing the adoption, which listed the twin boys' last name. They Institute said this was highly unusual. However they weren't shocked.
So next step is to get the formwork, certificates, and mail the request package out. I'm interested in learning why and when.
I'll keep you posted.
Eric Johnson, CA