Borna Bosnjak
The Holy Trinity brand bids its most legendary calibre goodbye with one final Royal Oak Openworked tribute in titanium and BMG.Limited to 150 pieces, its dial inspiration comes from a vintage pocket watch ref. 25729, and adds a new script logo to go with the other First Semester 2025 special editions.The end of the line for the 5135 architecture comes with a price tag of CHF 175,000.Audemars Piguet has become synonymous with the Royal Oak, but to history nerd and in-the-know connoisseurs, the brand will be just as closely tied with the perpetual calendar complication. Though not as pioneering a moment as introducing a leap year indicator, the Calibre 2120/2800 is likely the brand’s most historically important, seeing as it debuted in the ref. 5548 Perpetual Calendar at the height of the quartz crisis in 1978. It gave people enough time to start accepting the Royal Oak for the icon it was about to become, and bridged the gap it needed to become popular, and more importantly, sell and keep the brand afloat. Today, you’ll find the 2120/2800 in its most evolved form in the Calibre 5134 and 5135, the latter being a skeletonised variant made to show off the movement as much as possible, but their reign has come to an end, giving way to the new Calibre 7138. John Mayer’s signature Royal Oak signalled the end of the line for the 5134, and the new Royal Oak Perpetual Calendar Openworked 150th Anniversary is doing so for the 5135, with a titanium and BMG case and vintage pocket watch inspiration.The AP ref. 25729 that inspired the look of the 150th Anniversary Royal Oak Perpetual Calendar Openworked.Even before the complication made its way into watches, Jules Louis Audemars completed his horological education by making a perpetual calendar pocket watch (which also included a repeater and deadbeat seconds, by the way). Over the upcoming decades, AP would create calendar watches for its most exclusive clients, but just like its pocket and wrist-borne contemporaries from the likes of Patek Philippe, they all had one big flaw, and that was the lack of leap year indication on the dial. As a result, the watches would need to be sent to a watchmaker, the dial removed, and the movement set, making it highly impractical each time the watch stopped or was set incorrectly. Audemars Piguet solved that in 1955 with the ref. 5516, showcasing the leap year on the dial, with the help of Valjoux for the movement. What followed this nine-piece run (12, if you count the leap year-less pieces) was the JLC-based Calibre 2120/2800, with the ultra-thin Dubois Depraz perpetual calendar module that powered the Royal Oak ref. 5548. It is the movement that would form the basis of pretty much all AP perpetual calendars to come, would eventually become produced in house, and land in its current format as the Calibre 5134 and skeletonised Calibre 5135.The Royal Oak Perpetual Calendar Openworked (in any configuration) is just about as opulent a Royal Oak you can have without venturing into Offshore or Concept territory, but this 150th Anniversary model scales it back just a tad. AP has already done the brightly coloured ceramic thing in the past, but doesn’t skimp on material mastery here either, as it combines a 41mm titanium case with bulk metallic glass for the bezel, bracelet mid-links, and caseback. This amorphous metal alloy is not really a glass per se, getting its name from the ultra-shiny appearance it takes on as a result of rapid cooling. Audemars Piguet’s BMG compound is mostly palladium-based, with the white precious metal amounting for over 50% of the alloy mix, which boasts hardness superior to ceramic, meaning that it’s unlikely you’ll see it’s reflective surfaces marred with scratches.Audemars Piguet Ultra-Thin Perpetual Calendar ref. 5548. Image courtesy of The Time TraderOn the dial front, a sapphire blank shows off the Calibre 5135 inside, but I’ve gotta admit, the rest of its blue colourway is a pretty classy affair. The serif font throughout matches the anniversary Audemars Piguet script in the moonphase, which itself is a PVD-applied image of a NASA photograph of the Moon. AP note that it was inspired by the openworked Royal Oak-ish ref. 25729 pocket watch, though I could also see it as being a nod to the stunning platinum perpetual references 5558PT, a skeletonised variant of that all-important ref. 5548.And as cool as the 5135 is, it is also beginning to show its age. It beats at a very slow 2.75 Hz (though it doesn’t quite matter given it doesn’t have a seconds hand), with a power reserve of only 40 hours. That said, it’s not all about the on-paper performance specs, though you also have to mention the impressive height of just 4.5mm (9.9mm overall for the watch). The 5135 is a truly beautiful movement, and it takes to skeletonisation in a romantic way, shedding bits of metal wherever possible to show itself off fully, with numerous wide, hand-finished bevels the cherry on top.We’ve now seen the swan songs of both the 5134 and 5135 calibres, which were true (albeit upgraded) successors to the 2120, but if you’re really keen on an AP perpetual calendar based on this legendary movement, there’s still some hope. Audemars Piguet still makes the RD#2, an even slimmer variant, but one still undeniably based on the same movement. But with the advent of the new crown-operated Calibre 7138, I fear that time may be running out for the RD#2 as well.Audemars Piguet Royal Oak Perpetual Calendar Openworked 150th Anniversary pricing and availabilityThe Audemars Piguet Royal Oak Perpetual Calendar Openworked 150th Anniversary ref. 26585XT.OO.1220XT.01 is a limited edition of 150 pieces. Price: CHF 175,000 (US$195,000)BrandAudemars PiguetModelRoyal Oak Perpetual Calendar 150th Anniversary Openworked 41mmReference26585XT.OO.1220XT.01Case Dimensions41mm (D) x 9.9mm (T)Case materialTitanium and bulk metallic glass (BMG)Water Resistance20 metres, screw-down crownCrystal(s)Sapphire front and backDialSapphire, blue brass inner bezel, 18k white gold handsBracelet and strapTitanium and BMG bracelet, triple folding titanium claspMovementCalibre 5135, in-house, automaticPower Reserve40 hoursFunctionsHours, minutes, moonphase, perpetual calendarAvailabilityLimited to 150 piecesPriceCHF 175,000
26.02.25