I found this Montana Missing Persons Clearinghouse profile for Susan Elizabeth Pearson, noted as missing from Missoula since 1992. But when I ran her name through NewspaperArchive I found a bunch of articles from 1966 about a person by that name missing from Missoula. I’m pretty sure it’s the same one, too: both Susans are 5’0 and 97 pounds. And they disappeared on the same day, just not the same year.
I suppose maybe she disappeared twice, but that seems unlikely. I’m going with the 1966 date.