Thanks, as the uniform on the portrait I posted doesn't correspond to a real lancer uniform (neither Russian nor Prussian), I just take it as another indication of the credibility of the person depicted.
It is Luise Grafemus, who claimed to have been a Wachtmeister in a Prussian "lancer" regiment (this can only have been line Ulanen or Landwehr).
She found many people who believed her story and, and it seems she had even some cups with her portrait produced, to give to her supporters. Here her alleged Prussian uniform:
Her bio here:
I don't know what the Russian Wikipedia tells about her: