Here a video of where Luhmann's Zettelkasten is now and what it looks like, as well as some other stuff.
Attention: The language is German, but the pictures are interesting all by themselves.
From a 1988 Interview in Der Spiegel: "LUHMANN: Meine 26 Zettelkästen beruhen auf einem ausgeklügelten System von Verweisen. Die Zettel tun ihren Dienst willig und widerspruchslos. Doch es kann vorkommen, daß ich Zettel falsch einordne und dann nicht wiederfinde. Doch zu "Zettels Alptraum" ist es noch nie gekommen."
Asked whether he thought that Habermas also used a Zettelkasten, he answered: "Judging by his publications, Habermas orders his materials perhaps more by authors. Therefore he could perhaps do with a more simple system."
Anyone who has read "learning how to read" knows that this is for him the ultimate put-down.
About the computer: "For my ten meters of slips, covered with small (hand-)writing, computers come to late." This sounds somewhat melancholic (to me).
1. Luhmann: "My 26 Zettelkästen are based on a well-thought out system of references. The slips do their service willingly and without objections. Sometime I put a slip in the wrong place and cannot find it again. But a 'Nightmare of a Slip' has never happened yet" (Spiegel interview). The "Nightmare of a Slip" seems to be a reference to Arno Schmidt's Zettels Traum. For more information on this book, see here.
2. "Für meine zehn Meter dichtbeschriebene Zettel kommen die Computer zu spät."