The two movements of Prélude et Fugue AWV 107 were written at different periods of Alain’s life. The Fugue, numbered JA 57 by Marie-Claire Alain in the ‘JA Catalogue’, dates from 1935 when it was paired with a prelude, both movements for piano. The organ prélude paired later was numbered JA 75 in the JA Catalogue of manuscripts.
Alain played the work (piano version?) for Dupré at his home in Meudon in 1935. ‘After I had played him my latest work, Prélude et Fugue, I asked him if he found that Ducasse’s violent criticism was justified. He remained silent for an instant then announced to me that he would voluntarily sign my fugue!’ (Letter to Marguerite Evain, 22 October 1935).
In a letter to Evain dated 11 June 1938, Alain writes that ‘I would also like to put the finishing touches to a prélude and fugue for organ already finished for some time’.
In a letter to Daniel Lesur dated 5th January 1939, Alain writes that the ‘Modal Prélude et Fugue’… ‘has never been played in public’ and that it had been ‘composed in 1938’.
It seems that the three cadenzas (measures 11, 15 and 29) of the prélude were last to be included in the work. These appeared in an autograph copy which was presented as a gift to Marcel Dupré by the Alain family. Earlier manuscripts indicated their intended integration by the letters A, B, and C. The three cadenzas had been sketched out in a separate manuscript which Marie-Claire Alain reproduced in her 'Notes critiques' (2001).
The photograph on the video of Georges Caussade's (1873-1936) Counterpoint & Fugue Class of 1929/30 may be viewed (scroll up one) at:
http://musimem.com/CNSMP_classes.htm#1930
The score of the work for piano (listed simply as Prélude & Fugue) may be downloaded at:
https://imslp.org/wiki/Category:Alain,_Jehan
The 2002 Leduc score of the organ version is also listed (not Public Domain in the USA).