The Monthly Review (March 2020) contains an article ('Mystified Consciousness') by Andy Merrifield, who mentions:

Throughout the 1930s, Lefebvre worked on a companion volume to La conscience mystifiée called La conscience privée; the two texts were to be part and parcel of a larger project called Sciences des idéologies. Alas, the dialectical counterpart of the 1936 book was never completed. For years, Lefebvre’s manuscript, which he dedicated to Guterman—“reduced to silence on the other side of the Atlantic”—was thought destroyed. But it resurfaced in the 1950s and republished editions of La conscience mystifiée contain the fated text as an annex. A fascinating typescript of La conscience privée, bearing Lefebvre’s scribbled annotations, can be found at Guterman’s Columbia archives.

I hadn't realised this 'The private consciousness' text had already been published, apparently as an appendix in a French 1999 edition (Syllepse) of La conscience mystifiée. I don't have access to this 1999 edition to ascertain that the annex indeed contains this text. But supposing that Merrifield is correct, then it seems that Elden was unware of this annex to the French 1999 edition (although he cites it) in his Understanding Henri Lefebvre (2004, n. 45, p. 97), where he refers to it as if it were not published (initially titled as 'Essay on the individual'), and also to another project ('The time of the dupes'): "...and another, more political work entitled Le temps des dupes was also planned, along with a book on the notion of the individual. This last book, initially promised as 'Essai sur l'Individu' in their first piece in Avant-Poste (1933) would have treated Hume, Adam Smith and Keynes, and covered three areas:1. The glorification of the economic individual. 2. The contradiction between the theory and practice of individualism. 3. The 'triumph' of democracy and its difficulties." I'm of course intrigued by Elden's description of the contents. Trebitsch (1992) also noted these same two projects: "... ils comptaient poursuivre leur réflexion sur le thème de la mystification dans deux ouvrages, l'un intitulé Le Temps des dupes, tiré de leurs observations de la conjoncture politique, et surtout un autre, La Conscience privée, dont il reste d'importants manuscrits, qui partait d'une théorie critique de l''individu', contemporaine mais fondamentalement différente de la démarche personnaliste." If anyone has access to the 1999 edition then we can verify if Elden's claim that it treats Hume, Smith and Keynes, is true. Aside from this 'essay on the invidual' (or 'The private consciousness'), I learned that the Guterman Archive also contains an essay titled "Origine et définition de l'individu moderne" (although I hope this was published too in the 1999 edition, merging them together). As to the book 'The time of the dupes', there doesn't seem to have survived a manuscript of it in the Guterman Archive. It's also very interesting that Elden claims that there exists a book length manuscript by Guterman on 'The individual in the French novel', sometimes titled 'The myth of the individual as revealed by the French novel.' And it was confirmed to me that there does exist a 'treatise' on the novel in Guterman's Archive (although it was not communicated to me how many pages, or what precise notebooks). At this time (1930s), Guterman's co-thinker Lefebvre wrote the following 2 articles (online at Gallica), which are clearly on the same topic as Guterman's treatise: Essai sur les rapports de la critique et du roman, Commune , août 1937Stendhal ou le problème moderne de l'individu, Commune, mai 1939. As to Guterman's published works, there exist in English about 40 of his reviews in The New Republic in the 1930s, but not freely accessible online.