Don't make a mistake between Eric von Rosen and his son Carl von Rosen.
As far as I know, Eric von Rosen give a Thulin D (a copy of a Morane L) to the Finland. This aircraft became the first aircraft of the Finnish forces. His personal marking was a "Hakaristi" (a blue sun wheel or swastika). This symbol had nothing to do with the excecrated nazi symbol but was a lucky symbol. It was adopted as the Finnish air force marking until the end of the WWI act 2 (or if you prefer WW2).

If I remember correctly, Eric von Rosen never flew for the Germans or the Allies.
His son Carl (born in 1909 and a little too young to have flown in WWI

was the nephew of Karin Göring (the wife of Hermann Göring) and fascinated by the flying machines, he became a pilot (partly due to the influence of Hermann Göring during its career as a taxi pilot for Svenska Lufttrafik between the wars). During WW2, when the Russians attacked Finland, Carl von Rosen quit is pilot's job in the KLM, flying bombing missions for the Finnish forces. Married with a Dutch women, after the invasion of the Nederlands; he applied for the RAF but was refused on account of his families relations with Hermann Göring.
I hope this is of a little help.
Best regards from Belgium.
Edmond