The bronzes were among 12 animal heads that formed the centerpieces of an elaborate zodiac fountain and were carried off during the sacking of the old Summer Palace in Beijing by French and British troops in 1860 at the close of the Second Opium War.
The palace's buildings were burned and left in ruins as a punishment for the Qing emperor's obstinacy, and the bronzes spirited abroad into private hands.
China has made a priority of recovering them in recent years amid burgeoning pride in the country's economic achievements and a desire to reconstruct its former cultural and political glory. Five have already been returned to China and one is in Taiwan, but the whereabouts of four others remain unknown.
The symbolism of the bronzes derives from their origins in the reign of the Qing emperor Qianlong during the second half of the 17th century, a time when the empire's power, prestige and national territory were at their zenith.
Designed by the Italian Jesuit missionary Giuseppe Castiglione, they had been part of a rococco clepsydra, or water clock, placed in the forecourt of a pavilion inspired by Versailles.
The sacking of the palace, by contrast, stood as one of the major humiliations in the country's so-called century of shame that ended, according to the ruling Communist Party, only with the success of the revolution in 1949.
China's Cultural Relics Association estimates that more than 10 million cultural relics were taken overseas between 1840 and 1949, with many of them now displayed in museums in Europe and the U.S.