Boswell met a Prince of Hesse at the Hague on December 24, 1763. The Prince was in the army at the time, and Boswell met him at a parade at which he was on duty.
The editors of Boswell in Holland were not certain which Prince of Hesse Boswell met, but possible candidates seem to include the later Elector of Hesse-Kassel Wilhelm I (1743-1821) or one of the Landgrafs of Hesse-Kassel, Karl (1744-1836) or Friedrich (1747-1837). All three were sons of the Landgrave von Hesse-Kassel Friedrich II (1720-1785). One would think that the most likely candidate is the eldest, Wilhelm, as it seems to have been traditional only to refer to the hereditary Prince of Hesse as Prince, whereas his brothers were Landgrafs. Wilhelm married Princess Wilhelmine Caroline of Denmark in Copenhagen on September 1, 1764. However, when Boswell visited Hessen in 1764, he mentioned the hereditary prince specifically without referring in any way to them having met before. It is possible that he is referring to an entirely different branch of the royal family of Hessen, and that the title of Prince was used somewhat more liberally at the time.