In Memoriam: Josef Körner (9 May 1950) Robert L. Kahn The Modern Language Review, Vol. 58, No. 1 (Jan., 1963), pp. 38-59 https://www.jstor.org/stable/3720394

...of German letters long before Heine, impudently naive and imprudently honest, though always in- dustrious, 'griindlich' (and was it not Korner who 'discovered' the numerous un- published notebooks of Friedrich Schlegel and, in turn, left behind some twenty ' Zettelkasten ' which were recently acquired by Bonn University, a unique 'Fund-...

example of use of zettelkasten in English here in 1963 specifically as a loan word from German...