“Why was a man born in 1890 given an SSN in the 1970s?”
You can thank Jimmy Carter for that one, he signed into law The Social Security law that made it mandatory for all Americans to pay into the social security program.
It was the first attempt at saving a slowly dying system.
Also in 1977, new rules were put into effect regarding medicare requiring a SS number. Also, if any member of a household was receiving welfare benefits, ALL members of the household had to have a SS number and that had to be reported on the application for welfare benefits. This person born in 1890 may not have ever needed a SS number until then. If it was a female, she may have never worked. If it was a male, he may have worked for the Federal government or have been a railroad worker ( neither needed a social security number )