Rolex acquired the movement manufacturer Aegler in 2004.
Aegler was central to Rolex from the beginning. When Hans Wilsdorf and Alfred Davis founded Wilsdorf & Davis in London in 1905, they relied on Swiss movements supplied by the Aegler company in Bienne, Switzerland. Aegler specialized in compact movements suitable for wristwatches, which were still regarded skeptically by many customers at the time.
The relationship lasted for decades, but Aegler remained separately owned by descendants of the founding family. Its operation was later known as Manufacture des Montres Rolex SA and became closely integrated with Rolex production. In 2004, Rolex acquired the Bienne movement operation, completing a major step toward vertical integration.
The date is sometimes confused with 1905, when the commercial partnership began, or with 1919–1920, when Rolex moved its headquarters from London to Switzerland and registered its Geneva company. Those were important milestones, but they were not the acquisition.