I went to state college in the 90s and they posted test scores with SSNs. That is when I noticed that most of the students had one of two three digit prefixes. Thought it was based on when they were issued, looked up how SSNs worked and found out about the location pattern.

This was before the credit industry hijacked the number and started treating it as a secret code, despite SSNs not being intended for anything other than Social Security.