This is where it all began, ref. 5402. The 5402 Royal Oak is the watch which has most defined Audemars Piguet in the modern era. It foreran many trends we see today and despite an initially slow reception has gathered momentum with each passing decade. It is the start of integrated bracelets and considering the case-bracelet pair as a design. It was also the start of Genta's rather successful pen in that category. Finally, it began the modern era for Audemars Piguet, and went on to revive what was a then-struggling brand. In the perfectly phrased words of collector Juan Orozco, it is decidedly, 'different: imposing and aggressive, but still elegant'.

At debut in 1972, the 5402 was the most expensive steel watch ever made and by some margin. At 3750 CHF, it was more than four times the nearest most expensive steel watch from any other marque. Genta and Audemars Piguet were the first to treat steel as a luxury material: design first, metal second. It is a testament to AP's ability in watchmaking that they were able to execute on Genta's design so thoroughly, with an exceptionally intricate monocoque case construction, where the case middle and caseback are fused, with a front-loading calibre 2121 and eight bolts running through the case to meet screws on back (notably, those hexagonal bolts were only given symmetrically placed screw heads as decoration). It was then entirely hand finished with a beautiful contrast between delicate polished edges and an industrial brushed upper, echoed in the bezel and crown. Patek Philippe went on to be inspired by this very design, hiring in Genta to begin on the 3700 Nautilus a few years later. But the Royal Oak began the movement. Compared to a Nautilus, the design is more industrial, almost brutalist, but it wears with a comparable total elegance thanks to an ultra-thin profile.

At 39mm it was considerably larger than most offerings in its time, yet just 7mm thin. This made the 5402 blend immediately to your wrist and is responsible for most of its refined presence. This was thanks to an ultra-thin calibre 2121, a 19800-vph JLC 920 ébauche with gyromax balance and a beryllium rotor rail for less friction and wear. It is considered by many, one of the most refined movement architectures ever. In the words of historian Michael Friedman, the 920 'stands apart as one of the most reliable, beautiful and technically innovative movements of all time.' Moreover, for the 5402, AP were able to make the calibre 3.05mm thin as opposed to the standard's 3.4mm by removing central seconds. Hence, AP's end client was able to receive a watch true to Genta's design in what was one of the thinnest midcase profiles ever offered without sacrificing reliability.

A great degree of effort went in to creating a dial with a level refinement that could match the case and calibre. A few years earlier, Stern Frères had just inherited some old guilloché machines, and made suggestion to Genta that he use them in his efforts. Genta obliged, and Stern went above and beyond creating this almost woven 'petite tapisserie'. It is a particular style of multiple hand guilloché techniques, forming square pyramids with a diamond texture on top. As they were all turned by hand, there is immense variation in the size and number of pyramids between 5402 examples. Finally, it would be given a galvanic bath, originally a deep grey-blue. It was also the first use of the AP monogram at 12, applied in white gold.

This examples underscores its age beautifully in a uniformly tropical tone. Prolonged UV has created huge individual variance in 5402 dials, though they were varnished the grey-blue color was very prone to long-term evolution. This dial has a remarkable depth that comes alive in direct sunlight, due to contrast between lighter pyramid tops and darker channels between them as we often see in tropical 5402 dials. This is one of the earlier 'AP-up' dials, which first appeared in these C-Series cases. Swiss only.

The Royal Oak has evolved to include many varied cases and complications. In our opinion, the original design has yet to be bettered. It was a pioneering design, let alone retail strategy and engineering. The Royal Oak's importance to watchmaking and particularly AP as a brand today can probably not be overstated. The 5402, love or hate it, is easily one of the most significant watches of the last century.

This 5402 C-Series presents in great vintage condition. The calibre 2121 is beautifully in spec on our timing equipment, serviced just prior to sale. All mechanical functionality is perfect.

Its dial is an even rich brown, with varying levels of tropical patina on every individual square of the weaved 'petite tapisserie', with the pyramid's tops lighter than the deeper channels between them. Dial and hand tritium applications match and fade evenly under UV inspection. The level and effect of this tone is very dependent on light: we've tried to photograph this example in a range of scenarios. From bright light to interior, it can range from latte with a hint of purple to a warm grey. It really does have to be seen in person to be understood. This example has a correct 'AP up' dial with Swiss-only signature. Swiss Made, in C-Series 5402s, is indicative of a service dial replacement, which this is not.

Its case is beautiful, with proud even bevels. There is very light patina on its exterior, superficial surface wear and an almost imperceptible mark on the case lower at 9. The edges of this case are remarkably sharp with a tight bracelet, beautiful factory lines. The bezel top brushing may have been refinished, as bolts are a touch proud, but if so only slightly. It comes on a bracelet with full links and one extra, which will fit up to a 7.6" wrist (larger than most 5402s). Endlinks are correctly signed 'CXX'. Two of the extra lower-link bracelet pins show screw head wear, but function perfectly. The upper case pins show no wear at all. It is sporting is original unsigned crown, again not an AP-signed service replacement. It is numbered C176X, dating between 1978-1980.

This watch is modeled on a relatively large 7.7 inch wrist.

• Brand: Audemars Piguet
• Model: Royal Oak
• Reference: 5402ST
• Size: 39
• Year: 1978-1980
• Case Material: Steel
• Movement: Automatic Calibre 2121
• Scope: Watch Only