When Was the Portrait Painted?
The research team determined that the Delapierre portrait almost certainly was painted sometime between 17 May 1785 and 31 December 1785. They narrowed the date range as follows:
Therefore, it is highly unlikely that the book shown in the painting was a notional depiction by the artist of how he thought the book might look. Prior to the actual printing of the book on 17 May 1785, Delapierre would not have known what the unusual cover was going to look like.
So we can be confident that the portrait was painted sometime between 17 May 1785 and the end of the year.4
References and notes
 Three of the eight copies of De la Caisse d'Escompte purchased by the research team retain their original temporary paper covers. One of these temporary covers exposed the pages of the book in a manner virtually identical to the way the pages are exposed on the book in the painting. The two other original paper covers likewise were haphazardly affixed by the printer, exposing parts of the front and/or back pages. The five remaining copies had been rebound with permanent covers, but several of these had retained their ill-fitting original covers long enough to expose the outer pages to edge discoloration. And a copy of another pamphlet obtained by the research team—De la Banque d'Espagne, issued under Mirabeau's name and published a few weeks after De la Caisse d'Escompte, almost certainly by the same printer (Ref: Richard Whatmore & James Livesey, "Étienne Clavière, Jacques-Pierre Brissot and the intellectual origins of Girondin politics," Annales historique de la Révolution française, #321, July-September 2000)—has an ill-fitting cover and a label on the spine that are very similar to those shown in the painting (see picture below). All this evidence suggests that the ill-fitting covers were an idiosyncrasy of the particular printer who attached these temporary bindings, and therefore that the artist used a published version of De la Caisse d'Escompte rather than a notional copy as the model for the book in the painting.
 Antonina Vallentin, Mirabeau, translated by E.W. Dickes, The Viking Press, New York, 1948, p. 214. According to Vallentin, "...when Mirabeau wanted to print [De] La Caisse d'Escompte, he was able to do it in only the petty state of Bouillon, whence it was smuggled into France."
 We cannot entirely exclude the possibility that the painting was started and dated in 1785, but not completed until later.