Fun fact about PII:
People in the US military have a number but often they could use their social security number interchangeably. And it was common enough for forms to require one or the other that it just became standard procedure at one point to basically sign your name "FirstName LastName ###-##-####".
This was stopped eventually, but the National Climatic Data Center has /millions/ of pages of paper records of weather data with SSNs written on them.