What exactly did Luhman do and how did he do it? He wrote about 80,000 notes (Zettel) with cross-references between 1952 and 1996. Let us assume
- Note A was written in 1963.
- Note X was written in 1995.
On note X there is a reference to note A. How did Luhmann remember that there was a note A when he wrote note X 32 years later?
Many references are written with a different pen than the note itself. So they were added later. Where there are no references, nobody knows whether that was intentional or whether Luhmann only forgot them.
Edit: I forgot the conclusion 
What I’m actually trying to say is that any reference system is actually as good as Luhman’s. The individual pieces of information are linked … or not. Coincidence or intention?