Here is a digital copy of Luhmann's index of topics. It is relatively short and thus shows that he used the index to find significant access points into his note-taking system and not as an organizing principle for it. The organization itself was a result of his non-hierarchical designation of individual notes by numbers and letters.
The index has been typed for publication. Luhmann's Index, i.e. the list of the different numbers for main topics, looked like this:
This list is not identical with the list of access points or Schlagwortverzeichnis.
1. See Luhmann's Zettelkasten.
3. See also An Electronic Version.