When I worked in Oak Ridge in the 70s and 80s we had our SSNs stenciled on the back of our lab coats.
As personnel management became more sophisticated and formalized, a unique identifier was needed that was guaranteed to refer to one and only one person. And since SSNs already existed and were guaranteed unique, most of them just used those rather than inventing a unique identifier of their own.
Yeah.... about that: https://www.cio.com/article/3004596/a-tale-of-two-women-same...