A recent visit to the Musée de l'Armée in Paris brought me back in contact with a painting by him that the museum calls Remise de ses nouveaux drapeaux et étendards à l’Armée Française sur l’Hippodrome de Longchamp, le 14 Juillet 1880 (Web site citation here).
It is a large-scale study for a painting titled La distribution des drapeaux à Longchamp par le président Jules Grévy le 14 Juillet 1880 (link here) that Detaille chose to destroy after it had been exhibited. Apparently it hadn't been well-received, and Detaille also was somewhat dissatisfied with it. Some segments were cut out and later displayed as standalone works.
Readers interested in painters' techniques might wish to examine the photos I took of parts of the study version in the Musée de l'Armée. Detaille included an immense number of figures in the foreground and elsewhere, and readers can see how he indicated these. Click on my photos to considerably enlarge.