Catherina Elisabeth Wilhelmina van Nassau-LaLecqDied 1777
Life with Boswell:
Boswell knew her brother well (from Utrecht), but probably only met the Countess during his stay in the Hague in December, 1763. He dined with her at Baron von Spaen's on December 21, 1763.
This identification of the Countess de Nassau Ouwerkerke is almost certainly correct, although not a hundred percent. The Nassau family was (and still is) immense.