Dear Cuirassier,
You pose a very intersting question!
The painter of the portrait, Atala Varcollier (1803-1885) was the daughter of Constantin Stamaty, a Greek from Constantinople, a former French espionage agent who became the vice-consul of France at Civitevecchia in Italy. She was also the goddaughter of Chateaubriand and was named after the heroine in his novel "Atala". While in Italy in 1819, she married Michel-Augustin Varcollier (1795-1882), chef de la division des beaux-arts à la préfecture de la Seine, and the couple became close friends of Ingres. Atala is recorded as his most talented pupil in Rome and is known to have copied many of his works. They returned to Paris in 1820. Atala seems to have ceased painting after the death of their son Oscar (1820-1846).
The artist (seated at the piano) as sketched with her family by Ingres in 1818:
The artist in 1855, again by Ingres:
link
During the active years of Madame Varcollier's painting in Paris, the général comte Klein also lived ithere, at 87 r. de Bourbon / r. de Lille – about 600m from the Varcollier's hôtel at 8 r. Monthabor, on the other side of the Seine and through the Jardin des Tuileries.
Of the decoratons shown in the painting, the last awarded to the général comte Klein appears to be that of grand croix de la Légion d'honneur (29 April 1834). On the bar of decorations are miniatures of the Ordre de Saint Louis, the Légion d'honneur, and the Ordre du Lion de Bavière (Orden vom Pfälzer Löwen). The wearing of such miniatures was not uncommon during the era of the July Monarchy (1830-1848).
So, we might think that we could date the painting rather firmly to the years 1834-1844. But …. the général comte Klein was 71 years old in 1834, and looks much yonger in the painting.
Another depiction of the général comte Klein is an engraving published in 1823 by André-Ambroise Tardieu (1788-1841) – note again the youthful appearance of the subject, who was about 60 years of age.
It is possible that the portrait by Madame Varcollier was also done about 1821, then showing the blue and white sash of the Bavarian order. It could then have formed the baisis for the Tardieu engraving. Later, sometime after 1834, the portrait would have been overpainted with the red sash of a grand croix of the Légion d'honeur.
There is another oddity ….
The engraving by Tardieu gives a good rendition of the plaque or star of the Orden vom Pfälzer Löwen. But in the work by Madame Varcollier, the plaque or star placed lower on the left chest does not correspond well that of the Orden vom Pfälzer Löwen. The device in the painting looks a bit more like the star of a grand croix de l'Ordre de Saint-Louis, a dignity which (to my knowledge) was not accorded to the général comte Klein.