Wondered with @SarahAMcManus "whence index cards?"
Found this excellent article! Relevant to @Conaw's interests, it cites an early mail-in search engine project that would return index cards! 
Founder gave up but envisaged eventual digital solution.https://www.popularmechanics.com/culture/a19379/a-short-history-of-the-index-card/ …
There’s a certain type of librarian that transitioned smoothly into all our database admins, it’s exactly the same job.
