Known as the "Bao Dai" watch, this piece was sold to the last emperor of the Nguyen dynasty in Vietnam in 1954.
It's a Rolex reference 6062, and was both the most expensive and rarest watch the brand sold at the time. The gold case and black dial were the rarest combination of the 6062 model.
Only three models to this specification are known to have existed, according to Hodinkee. This particular model is the only one to have had diamond markers on the even hours, making it completely unique.
It was sold by the Nguyen family for $235,000 in 2002, meaning that the watch has only changed hands once. Though that price may seem low compared to today's standards, it was the most expensive Rolex ever sold at the time. Experts think it may claim that title again.
Its auction estimate is $1.5 million, though many collectors think that the watch will actually go for much more than that when the hammer falls, according to Hodinkee. The current record holder is the Rolex split-seconds chronograph reference 4113, which sold for $2.5 million last year.