If a more extensive note has been put on several A6 sheets subsequently,
Heydes Overbearing System
Heyde's method spends almost a full page talking about what to do if one extends a note or writes longer notes. He talks about using larger sheets of paper, carbon copies, folding, dating, clipping, and even stapling.
His method seems to skip the idea of extending a particular card of potentially 2 or more "twins"/"triplets"/"quadruplets" which might then also need to be extended too. Luhmann probably had a logical problem with this as tracking down all the originals and extending them would be incredibly problematic. As a result, he instead opted to put each card behind it's closest similar idea (and number it thus).
If anything, Heyde's described method is one of the most complete of it's day (compare with Bernheim, Langlois/Seignobos, Webb, Sertillanges, et al.) He discusses a variety of pros and cons, hints and tips, but he also goes deeper into some of the potential flaws and pitfalls for the practicing academic. As a result, many of the flaws he discusses and their potential work arounds (making multiple carbon copies, extending notes, etc.) add to the bulk of the description of the system and make it seem almost painful and overbearing for the affordances it allows. As a result, those reading it with a small amount of knowledge of similar traditions may have felt that there might be a better or easier system. I suspect that Niklas Luhmann was probably one of these.
It's also likely that due to these potentially increasing complexities in such note taking systems that they became to large and unwieldly for people to see the benefit in using. Combined with the emergence of the computer from this same time forward, it's likely that this time period was the beginning of the end for such analog systems and experimenting with them.