Audemars Piguet celebrated an important milestone at SIHH 2018 - the Royal Oak Offshore turned 25 this year – but this wasn’t the big newsmaker for the Le Brassus brand at Geneva. That honor went to a revolutionary new RD#2 Concept Royal Oak Ultra Thin Perpetual Calendar.

Moonphase at 12 is a nod to the first AP perpetual calendar wristwatch

When the concept watch finally goes into production, at just 6.3 mm thin, this will be hands-down the thinnest perpetual calendar watch in the market. The Royal Oak Concept collection, introduced in 2002 sees the haute horology brand take a leaf from the automotive industry’s book by introducing one-off concept models. The concepts, demanding on the market demand, finally make it into regular production line. Past concept models include the Laptimer and the Super Sonnerie (which was born as the RD#1 Concept Acoustic Research Minute Repeater).

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This new concept brings together two traits that the brand is known to do very well – create ultra slim watches and perpetual calendars. Audemars Piguet has a great tradition of producing perpetual calendars and in 1955, it produced the Ref. 5516, the first perpetual calendar wristwatch with a leap year indication.

It took five years to develop the new Caliber 5133

The RD#2 uses a 41-mm-wide 950 Platinum case which is only 6.3 mm thin thanks to the five years of R&D that went into developing the new ultra-thin Caliber 5133. It might serve us a well to go back in time to understand the significance of what AP’s engineers have done here.

In the 18th century, the usual practise was to have the movement between two parallel plates and the balance wheel outside the top plate in the construction of a pocket watch movement. It was Jean-Antoine Lépine who discarded the top plate with a new kind of movement. The Lépine calibre discarded the top plate and used individual cocks mounted on a single plate.

At 6.3 mm thin, this is like no other perpetual calendar

Instead of the traditional frame with two bottom plates, this calibre used a single plate onto which the train is fixed with independent bridges. The traditional fusée-and-chain transmission was swapped for a cylinder escapement. Lépine calibre would herald the creation of slimmer pocket watches and watch companies have continuously pursued the development of slimmer movements since.

Caliber 5133 is based on the Caliber 2121 used in the Royal Oak Extra Thin. The challenge on the 5133 was to reduce the number of stacks on the movement from three modules to one. The pursuit of engineering a single level movement from a three-storey one while keeping it efficient and robust was not an easy one. It took them five years of research and development to come up with the required solution. The movement has a 40-hour power reserve and beats at 2.75 Hz (19,800 vph).

The RD2 is 6.3 mm compared to the existing QP model at 9.5 mm

This patented system features a record-shattering 2.89mm central rotor. At just 6.3 mm, the redesigned case shaves almost 2mm off the Royal Oak Extra-Thin Jumbo, making the Royal Oak RD#2 the thinnest self-winding perpetual calendar on the market today. The existing Royal Oak perpetual calendar is 9.5 mm thin.

The blue “Grande Tapisserie” guilloche dial has a slightly different arrangement to the regular perpetual calendar thanks to a bigger dial (remember this is a 41 mm case). The moonphase is now placed closer to 12 o’ clock as a nod to the first perpetual calendar wristwatch produced by the brand in 1955. There is no word on when the model will go into regular production, but considering how quickly the Super Sonnerie made it as a production model (a year after the concept was shown at SIHH 2015), it may not be long.

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