(Stamp Hanover, 1856)
The letter was send to an official of the Franch state: someone with the rank of a minister.
What was the political situation in Hanover 1865? After Napoleon I imposed the Convention of Artlenburg (Convention of the Elbe) in 1803, about 30,000 French soldiers occupied Hanover. The Congress of Vienna in 1815 elevated the electorate to the Kingdom of Hanover. In 1837, the personal union of the United Kingdom and Hanover ended as William IV's heir in the United Kingdom was female (Queen Victoria).
(The House of Hanover - 15 September 2011 - stamps and miniature sheet. The penultimate Kings & Queens issue covers the Hanoverian era from 1714 to 1901).
According to Salic Law Hanover could only be inherited by males.
As a consequence, Hanover passed to William IV's brother, Ernest Augustus, and remained a kingdom until 1866, when it was annexed by Prussia during the Austro-Prussian war.
Gustave Armand Henri, Comte de Reiset, 1821-1905 was a "ministre plénipotentiaire" of Napoleon III of France in Hanover.
(Stamp Napoleon III)
De Reiset was ennobled by King Louis-Philippe and adopted the "devis" : "Faire sans Dire" (do without saying). He was also member of the Conseil General of the Eure, a region in France. He owned a castle there.(Photos: French Ministère de la culture - base Mémoire)
It was classified as an Historical Monument in 1993 and acquired in 1995 by a couple who then started the restoration. It cannot be visited today.
You can google some artefacts that formerly belonged to Mr. de Reiset, such as this portrait of the famous painter Ingres, this painting is entitled Madame Aymon or "La Belle Zelie".
I have a second cover of a letter written to the same Comte de Reiset. It was sent in 1862 from Frankfurt to Darmstadt.
I do not know about where these covers come from and who took interest to keep them all these years. I bought them in the US, but no story came along with them.