If you’re generalizing Zettelkasten to “All Non-Linear Knowledge Management Strategies” You should include Mortimer Adler and the Syntopicon, and John Locke’s guide to how to set up a commonplace book
Let’s be honest that these are some of the lowest hanging fruit on the tree of this incredibly deep history.
Adler as an encyclopedist was assuredly more than aware of the commonplace tradition and likely knew or read any/many of the following:
* Rodolphus Agricola. De formando studio (written 1484, published 1508)
* Dediderius Erasmus. De ratione studii (1512) and De duplici copia verborum ac rerum (1512)
* Philp Melanchthon. De locis communibus ratio (1539)
One could certainly say that Adler had a Zettelkasten of his own or at least a shared one.
These were instrumental in popularizing the idea of the commonplace book not only in the Renaissance, but firmly placed them in the foundations of education for the coming centuries.