It's been reported by his son that Luhmann did learn about a method (which either he heavily modified or someone else showed him their modification thereof) from Johannes Erich Heyde. There's sure to be more details on this in Scott Scheper's upcoming book Antinet Zettelkasten.

see: Heyde, Johannes Erich. Technik des wissenschaftlichen Arbeitens: zeitgemässe Mittel und Verfahrungsweisen. Junker und Dünnhaupt, 1931.

See: https://hyp.is/4wxHdDqeEe2OKGMHXDKezA/www.reddit.com/r/antinet/comments/wryt4t/the_secret_book_luhmann_read_that_taught_him/