<small><cite class='h-cite via'>ᔥ <span class='p-author h-card'>Manfred Kuehn</span> in Taking note: Luhmann's Zettelkasten (<time class='dt-published'>08/06/2021 00:16:23</time>)</cite></small>

Note the use of the edge highlighted taxonomy system used on these cards:

Similar to the so called high five indexing system I ran across recently.

https://www.flickr.com/photos/hawkexpress/albums/72157594200490122/

https://lifehacker.com/the-pile-of-index-cards-system-efficiently-organizes-ta-1599093089

LifeHacker covers the Hawk Sugano's Pile of Index Cards method, which assuredly helped promote it to the GTD and productivity crowd.

One commenter notices the similarities to Ryan Holiday's system and ostensibly links to https://thoughtcatalog.com/ryan-holiday/2013/08/how-and-why-to-keep-a-commonplace-book/

Two others snarkily reference using such a system to "keep track of books in the library [,,,] Sort them out using decimal numbers on index cards in drawers or something..." and "I need to tell my friend Dewey about this! He would run with it." Obviously they see the intellectual precursors and cousins of the method, though they haven't looked at the specifics very carefully.

One should note that this may have been one of the first systems to mix information management/personal knowledge management with an explicit Getting Things Done set up. Surely there are hints of this in the commonplace book tradition, but are there any examples that go this far?

Interesting to see the first post on zettelkasten.de in 2013 has a photo of hawkexpress' Pile of Index Cards (PoIC) in it.

This obviously means that Christian Tietze had at least a passing familiarity with that system, though it differed structurally from Luhmann's version of zettelkasten.