I have what I think is a direct, and perhaps quicker solution.
Since you (the Sodder family) have the postmark from the original envelope, and we know this is a compelling story, I would interest the local paper in that area in printing the picture and doing a "Have You Seen This Man?" story, explaining what happened.
Someone in that town will know who that is (or was). Most people from 1968 will still be alive. I also looked into this once and most people do not leave their hometowns. A ridiculously high number of people who lived there in 1968 will still be there. He is also a good looking young man. Someone is going to remember that face.
If by any chance the paper does now want to pursue this, which I don't think is likely AT ALL, or you would rather not have a piece like this done, you could take out an ad and print it yourself.