The eighteenth and part of the nineteenth century had still made do with notebooks and catalogs. These methods were succeeded by the card index, which did not begin to be properly organized until the early twentieth century. In its most rudimentary form, it already represents a real exteriorized cerebral cortex. A simple set of bibliographical index cards will lend itself to many adaptations at the hands of its user, becoming a an author or subject card index, a geographical or chronological one with every possible permutation to meet requirements as particular as the place of publication or the dimensions of inset plates. This is still more obvious in the case of card indexes containing scientific information, where each documentary component can be rearranged at will in relation to all other components. Actually, the image of the cortex is in some respect misleading, for if a card index is a memory in the strict sense, it is a memory lacking its own means of recollection and has to be brought into the researcher's visual and manual operational field before it can go into action. (André Leroi-Gourhan 1964/65 //Le Geste et la parole//, quoted according to p. 263f. of the English translation (Gesture and Speech. Cambridge: MIT Press, 1993.)I would find it difficult not to read Luhmann's claim as a direct response, variation, and further development of Leroi-Gourhan's claims about the exteriorizsation of memory and the similarities of the card index to an external cortex.
Leroi-Gourhan' swork deserves to be better known outside the circles of prehistory. I only came across it fairly recently.