I now have definitive written proof of a numeric indexing system (delineated in a popular textbook for secretaries, and published by a company which did significant business in Germany) from the mid-1920s with increasingly more detail into the 1940s and a fifth edition published in 1950. It’s exactly like, but notably predates, Niklas Luhmann’s alphanumeric system which he started in 1952. I’ll write up some of the details shortly with specific references, but thought I’d tease it here a bit first.