It's not a SSN but a health insurance number, which is basically structured like this: three first letters of your lastname, the first letter of your first name

It's not a SSN but a health insurance number, which is basically structured like this: three first letters of your lastname, the first letter of your first name, the two last digits of your birth year, the month in two digits format of your birthdate, the day in two digits format of your birthdate and a somewhat random number between 00 and 15 IIRC. So if you know someone named John Doe and their birthdate (ex: October 11th 1951), you can mostly guess their health insurance number (ex: DOEJ 5110 11xx). Not exactly rocket science.