For those who’ve been waiting for it to show up, the digitized version of box 9 of Niklas Luhmann’s second zettelkasten is now available online. It contains many of his notes on the zettelkasten itself.
https://niklas-luhmann-archiv.de/bestand/zettelkasten/zettel/ZK_2_NB_9-8g_V
https://niklas-luhmann-archiv.de/bestand/zettelkasten/zettel/ZK_2_NB_9-8g_V