HMS William of Orange
From TemeraireWiki
William of Orange was a dragon transport ship in the Royal Navy similar to the HMS Allegiance. It arrived from Halifax just after the Valerie engagement, providing needed supplies to the Allegiance and men for Maximus's crew. The purpose of this voyage was to the dragon Dakota from the Americas to British breeding grounds.
Presumably, William was therefore the ship that carried Praecursoris to Halifax in exchange for Dakota.