Replied to a thread by Scott P. Scheper and Alexandra Graßler (Twitter)

The origins of the Zettelkasten's numeric-alpha card addresses seem to derive from Niklas Luhmann's early work as a legal clerk. The filing scheme used is called "Aktenzeichen" - See https://t.co/4mQklgSG5u. cc @ChrisAldrich
— Scott P. Scheper (@ScottScheper) June 27, 2022

The "Aktenzeichen" was the first which came to my mind as I stumbled across the Zettelkasten years ago. And it makes total sense for me. It's the most important signature of every kind of bureaucracy here in Germany and Luhman worked for years in this field.
— Alexandra Graßler (@wissensagentur) June 28, 2022

I’ll go you one better with the likely historical precedent for Niklas Luhmann’s zettelkasten numbering system: conscription numbers on Viennese houses in 1770s!
Alberto Cevolini & Markus Krajewski have relevant research. There are still a few missing puzzle pieces however.