reply to harr at https://forum.zettelkasten.de/discussion/3392/folgezettel-vs-duplex-numeric-arrangement
I'll shortly have a lot more to say on this very subtle historical subject, which I've been work at off and on over the past month or so. My analysis indicates entire lack of innovation on the fronts which you're indicating. Pages 178-180 show the period standard practice of the subject alphabetic filing you say Luhmann was innovating against, but the duplex-numeric is exactly what he was using. The method he chose had been recommended and in use since at least the 1910s—especially for law offices.
Your quotes from his 1981 paper, while interesting, create a false impression stemming from post hoc, ergo propter hoc analysis. You have to remember that by the 1980s, he's been practicing this for nearly 30 years and is providing a reflection on that practice, which is also heavily impacted by his systems theory work through those decades. I strongly suspect that his mid-century perspective didn't stray far from that Remington Rand outline or those of scores of other sources.
It bears noting that of the four potential methods suggested in the chapter, the last one is the Dewey Decimal method, which many who've been in the zettelkasten space have also very naturally tried using as a scaffolding for their filing work. Others have also reasonably suggested variations like the Universal Decimal Classification system or Wikipedia's Academic Outline of Disciplines.
One will also notice that the option of doing a "Variadex Alphabetic" arrangement hasn't ever (to my knowledge) been mentioned in the online zettelkasten space. It was given the pride of place as first in the list of options, but this stems primarily from the fact that it was a variation offered by Remington Rand as a paid product with the related accessories. Every filing cabinet company and major stationery company had variations on this theme with their own custom names and products.