It's not a ZK furniture though. Index cards were not used to store atomic notes, or have alphanumeric indexes. :)

They would store communique, contact information, business transactions, invoices. The (early) 1900's was an interesting time for businesses - with the telephone becoming more standard, and the advent of the automotive.

Index cards were very popular for businesses during this time. Easy to transport, easy to file away, look up, etc. Libraries used it as a standard feature (I'm sure you're all familiar with the dewey decimal system). Records, invoices, customer data were stored on these index cards due to how durable they were compared to normal paper.

The most popular size of the time was the 5" x 8" index cards (owing to the emerging popularity of the Kardex filing systems), which is probably what this cabinet accommodates! This is approximately the A5 paper size.

With the advent of digital computers, businesses moved away from these index cards and back to cheaper printed paper.

The most common size nowadays is 3" x 5".

Edit: as an aside, I'm surprised no one has ever talked about Thomas Harrison's or Vincent Placcius' Arca Studiorum. I mean, Vincent was practically an early alpha-beta version of Luhmann, before Luhmann was even born. (https://en.wikipedia.org/wiki/Vincent_Placcius)