Ask HN: How did the pros manage source code decades ago before modern SCM?

Always wondered how, in the days before SVN and Git and others, like in the 1970s - 1990s, during that enormous golden age of office software, video game software, etc, how were developers managing source code changes and versioning? I started hobbying at the tail end of that era and just used different copies of files in different folders for "versioning" so I could "revert" by going back to a different folder copy.