An exceptionally rare and unique Rolex is set to go up for auction with Phillips in May and collectors are being told to count their pennies.

The watch, known as the ‘Bao Dai’, was sold to the last emperor of the Nguyen dynsasty 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 model comes with a gold case and black dial were the rarest combination of the 6062 model.

According to Hodinkee, only three models to this specification are known to have existed. This mode is the only one of its kind to have had diamond markers on the even hours, making it completely unique.

In 2002 it was sold by the Nguyen family for £235,000 which meant that the watch has only changed hands one. At the time, this was the most expensive Rolex ever sold.

It’s estimated that the watch will go to auction for $1.5 million but collectors are anticipating the watch will go for much more. The current record holder is the Rolex split-seconds chronograph reference 4113 which sold for $2.5 million in 2016.