The bronze Wenlok jug, pictured here, is thought to have been made for either William Wenlock, who died in 1391 and was canon of St Paul's Cathedral, or his great-nephew John, the first Lord Wenlock and a major figure in the 15th century. Both had strong connections to the Luton area.
In 2005 it was bought by London dealer Daniel Katz for a premium-inclusive £568,000 against an estimate of £60,000-80,000, but was barred from export by the Department of Culture on the grounds of its outstanding significance to the study of bronze working in medieval England. The jug had caught the eye of New York's Metropolitan Museum.
Luton Museums Service raised the money to buy it after a deadline was extended and it was then put on display at the Stockwood Discovery Centre (the former Stockwood Park Museum), which covers the history of the area.
The 12½in (31.5cm) tall jug was taken from a high-security display cabinet during a burglary on Saturday, May 12, between 11 and 11.25pm.
It is cast with a number of insignia, including the Royal arms as used between 1340 and 1405, a maker's mark and the inscription To My Lord Wenlok.
Anyone with information should call Bedfordshire Police on 101 or text details to 07786 200011, or call Crimestoppers anonymously on 0800 555111.