The International Association of Amateur Heralds

George Keith was the 10th and last Earl Marischal of Scotland, hence the reference to "Maréchal d'Ecosse" though having been a Jacobite he had lost both lands and title and had to make his career in exile. He served as Frederick the Great's ambassador to Spain from 1759 to 1761 and during that period was pardoned by King George II. Neuchâtel was then a possession of the Kings of Prussia and Keith served as its Governor, some sources say from 1752 but the armorial painting suggests 1754-68.

https://www.jamesboswell.info/biography/george-keith-10th-earl-marischal