By Constantin Carathéodory. Rendiconti del Circolo Matematico di Palermo, volume 32, pp. 193–217 (1911).
Publisher metadata: link.springer.com/article/10.1007/BF03014795
The translation is titled On the Domain of Variability of Fourier Coefficients of Positive Harmonic Functions and credits Nicholas J. Williams as translator. It records the original announcement on 26 March 1911 and printing on 16 August 1911; neither dates the translation. The original German scan was not accessible.
Cited passages: Introduction §4, original printed p. 195; §I opening.