Interesting choice of translation for "Die Kartei" by the translator. Some may have preferred the more direct "file".

Historically for this specific time period, while index cards were becoming more ubiquitous, most of the prior century researchers had been using larger sheets and frequently called them either slips or sheets based on their relative size.

Beatrice Webb in 1926 (in English) described her method and variously used the words “cards”, “slips”, “quarto”, and “sheets” to describe notes. Her preference was for quarto pages which were larger pages which were likely closer to our current 8.5 x 11” standard than they were to even larger index cards (like 4 x 6".

While I have some dissonance, this translation makes a lot of sense for the specific time period. I also tend to translate the contemporaneous French word “fiches” of that era as “sheets”.

See also: https://hypothes.is/a/OnCHRAexEe2MotOW5cjfwg https://hypothes.is/a/fb-5Ngn4Ee2uKUOwWugMGQ