One of the rarest watches in the world may become the most expensive Rolex ever sold

Rolex collectors, start counting out your pennies. An exceptionally rare and unique vintage Rolex watches will go up for auction with Phillips in May.

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.

Rolex Sea-Dweller Resurfaces with Larger Case and New Movement

It’s another big year of anniversaries in the luxury watch world. While Omega marks 60 years of the Speedmaster and Patek Philippe, 40 years of the Aquanaut, Rolex celebrates the half-century mark for its extreme divers’ watch, the Oyster Perpetual Sea-Dweller, by launching an all-new model, with a larger case and modern caliber, at Baselworld 2017.

The original Rolex Sea-Dweller, created in 1967, was designed as a resilient and useful tool for professional deep-sea divers of that era. Among its many notable features was a helium escape valve, patented by Rolex that same year, which preserved the watch’s water-resistance while regulating the air pressure accumulated inside its case during the decompression phases of deep-water saturation dives.

The case of the new Sea-Dweller is 43 mm in diameter — 3 mm larger than its 40-mm predecessor. The scratch-resistant sapphire crystal over the deep black dial is equipped, for the first time on this model, with a Cyclops lens over the date window at 3 o’clock, enhancing its legibility. The text “Sea-Dweller” appears on the dial in red, echoing the look of the original 1967 model. Finally, the watch is equipped with the used Rolex Caliber 3235, a self-winding movement boasting a number of innovative technical details, some of them patented. This represents the first time Rolex has employed this recently introduced, technically superior movement in one of its “Professional grade” timepiece. Like all used Rolex watches for sale, this Sea-Dweller carries the Superlative Chronometer certification, instituted by Rolex in 2015, which ensures a high level of precision and timekeeping performance (-2/+2 seconds per day).

Like its most recent predecessors, the new Sea-Dweller is water-resistant to a depth of 1,220 meters (4,000 feet). Its 60-minute graduated, unidirectional, rotating divers’ bezel is fitted with a patented black Cerachrom bezel insert manufactured by Rolex in a virtually scratchproof ceramic whose color is unaffected by ultraviolet rays. The bezel’s graduations are PVD-coated with a thin layer of platinum. The dial’s large hour markers are filled with Chromalight, a Rolex-developed luminescent material that emits a long-lasting blue glow in low-light conditions.