Theodor Mommsen's zettelkasten
My German is less than proficient, but it appears that the following source credits historian Theodor Mommsen (1817–1903, author of the multivolume History of Rome, a work which was influential for his 1902 receipt of the Nobel Prize for literature) with a zettelkasten of 75,000 slips.
Rebenich, Stefan. “Theodor Mommsen and the Relationship of Age History and Patristic (Theodor Mommsen und das Verhältnis von Alter Geschichte und Patristik).” In Patristique et antiquité tardive en Allemagne et en France de 1870 à 1930 : Influences et échanges: Actes du colloque franco-allemand de Chantilly (25-27 octobre 1991), edited by Jacques Fontaine, R. Herzog, and Karla Pollmann, 131–54. Paris: Brepols, 1993. https://archiv.ub.uni-heidelberg.de/propylaeumdok/46/1/Rebenich_Mommsen_1993.pdf.
Perhaps others with more facility with the language may correct me or be able to find the library or museum which may have the remaining portions of the collection now. Given the dates, one might presume his collection was ordered topically rather than in the Luhmann-artig fashion.
My rough machine translation of the relevant section on page 144:
The 84-year-old Mommsen had thus initiated a project on which both profane and church historians in Germany continued to work long after his death and which made good progress: Al s Jülicher, who took over the sole management after Seeck's death in 1921 due to his eye problems in 1929 was relieved of his obligations, he handed over around 75,000 slips of paper to the Commission, the systematisation of which, however, was not completed. The material was not published because "neither the condition of the manuscript nor the scientific situation, which had meanwhile completely changed, allowed the prosopography to be printed".
After only a few card boxes had been lost in World War II, some of the materials were loaned to A.H.M. Jones, John Morris and H.-I. Marrou in Cambridge and Paris, which had set itself the goal of developing a secular and an ecclesiastical prosopography of late antiquity. While today, for technical and organizational reasons, we have to work with two different prosopographies - one secular and one ecclesiastical - both of which have not yet been completed, Mommsen recognized the usefulness, indeed the necessity, of a late antique prosopography that explores secular and ecclesiastical dignitaries together and performs.