Hi Chris, I checked Chavigny's book on the BNF site. He insists on the use of index cards ('fiches'), how to index them, one idea per card but not how to connect between the cards and allow navigation between them.

Mind that it's written in 1919, in Strasbourg (my hometown) just one year after it returned to France. So between students who used this book and Luhman in Freiburg it's not far away. My mother taught me how to use cards for my studies back in 1977, I still have the book where she learn the method, as Law student in Strasbourg "Comment se documenter", by
Roland Claude, 1961. Page 25 describes a way to build secondary index to receive all cards relatives to a topic by their number. Still Luhman system seems easier to maintain but very near.

I'm not a fan of ZK myself. It was great before computers and Internet but it's a lot of work and adds a lot of friction.