The firm, which also has 550 safe-deposit boxes on site and room for about 450 more, is betting on London's reputation as a "safe haven," even with Brexit.
There are more than 25 million safe deposit boxes by some estimates in the U.S. alone.
A private collector held the Crown of the Andes, made with 5.3 pounds of gold and more than 400 emeralds, in a Citibank box before its sale four years ago to the Metropolitan Museum of Art.
Demand has waned in recent years, according to JPMorgan Chase & Co. and Bank of America Corp., the nation's two largest lenders.
UBS Group AG, the world's biggest wealth manager, and rival Credit Suisse Group AG announced plans this year to expand measures for charging rich clients for holding excess cash. To some, these extra costs have made the safe deposit box a viable alternative. A spokeswoman for UBS' Switzerland bank, which operates almost 250,000 safe deposit boxes nationwide, said demand from clients has declined in recent years.
Failing to strictly adhere to a safe deposit firm's protocols may result in being rejected as a customer.
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