The Audemars Piguet Royal Oak RD#2 was a 2018 research-and-development prototype that showed how a selfwinding perpetual calendar could fit in a 6.3 mm case. Its key was not simply making every component smaller: AP rearranged the calendar into a single-level mechanism. The platinum prototype’s slimness was striking, but so was its heft, according to Hodinkee’s Jon Bues, who emphasized that his impression was not a measurement.
What was the Royal Oak RD#2?
Audemars Piguet presented the RD#2 at SIHH in January 2018 as a prototype exploring an ultra-thin automatic perpetual calendar. The name refers to a research-and-development project, not the production reference that followed. AP’s heritage chronology records the 2019 Royal Oak Selfwinding Perpetual Calendar Ultra-Thin, reference 26586IP, as a 41 mm platinum-and-titanium model. Those production materials should not be confused with the prototype’s all-platinum case, bezel and integrated bracelet.
AP’s official record gives the RD#2 a case height of 6.3 mm and its selfwinding Calibre 5133 a thickness of 2.89 mm. AP described the calibre as breaking the thinness record at the model’s release; that is a historical claim, not evidence that it remains the current record. See AP’s AP Chronicles and its history of Royal Oak calendar watches.
How did AP make a perpetual calendar this thin?
A perpetual calendar tracks the differing lengths of months and the leap-year cycle, rather than requiring the wearer to correct the date at the end of every short month. AP Heritage and Museum Director Sébastian Vivas describes the complication as “a small mechanical computer on the wrist capable of keeping the date correct for an entire century.” That is a description of the complication’s intended calendar cycle, not a claim that every perpetual calendar never needs adjustment under all conditions.
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The RD#2’s engineering answer was to bring the calendar’s functions together on one level. AP’s overview calls out the single-level architecture; Hodinkee’s contemporaneous explanation describes a change from three stacked layers to a flatter arrangement of interacting parts. Its example is a 48-step month wheel: notches of different depths encode months of different lengths, with the leap-year February included in the sequence. The report also says the dial serves as a structural plate supporting the movement. This arrangement helped achieve a thin calibre without treating the calendar as a conventional stack that simply had to be compressed. AP summarizes the architecture in Haute Horlogerie.
What did the 2018 prototype feel and look like?
In his January 2018 SIHH hands-on report, Jon Bues described the 41 mm prototype’s platinum case, bezel and integrated bracelet as unusually heavy in the hand despite the thin profile. He explicitly said he had no scale, and AP had not published a weight, so this is an observer’s impression rather than a measured specification. Bues also noted how closely the dial and crystal sat together, emphasizing the dial pattern and the apparent depth of the displays.
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The distinction between impression and specification matters here: the 6.3 mm case height is documented, but no weight figure is established for the prototype. Bues’s account is in Hodinkee’s RD#2 hands-on report.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Where does Calibre 5133 fit in Royal Oak history?
AP traces Royal Oak calendar complications to 1983–84. In its chronology, the 2.89 mm Calibre 5133 is compared with the earlier Calibre 2120/2800, which measured 3.95 mm thick. That comparison helps show the scale of the movement-thickness achievement, while the RD#2’s 6.3 mm figure describes the complete case rather than the calibre alone. The figures come from AP’s Royal Oak calendar-watch history.
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RD#2 prototype versus production reference 26586IP
| Detail | RD#2 prototype (2018) | Reference 26586IP (2019) |
|---|---|---|
| Status | Research-and-development prototype presented at SIHH in January 2018 (AP Heritage / AP Chronicles) | Production Royal Oak Selfwinding Perpetual Calendar Ultra-Thin identified in AP’s 2019 chronology (AP Heritage / AP Chronicles) |
| Case diameter | 41 mm, as reported in Jon Bues’s 2018 hands-on account | 41 mm (AP Heritage / AP Chronicles) |
| Materials | 950 platinum case, bezel and integrated bracelet (Jon Bues, 2018) | Platinum and titanium (AP Heritage / AP Chronicles) |
| Case height | 6.3 mm (AP Heritage / AP Chronicles) | not stated in the cited AP chronology |
| Movement | Selfwinding Calibre 5133, 2.89 mm thick (AP Heritage / AP Chronicles) | not stated in the cited AP chronology |
What the hands-on account does not establish
- It does not provide a measured weight for the prototype; “heavy” is Bues’s qualitative impression.
- It does not establish the prototype’s present-day sale availability or a current price. The 2018 report’s description of its status applies to that time only.
- It does not establish that Calibre 5133 still holds the world’s thinnest perpetual-calendar record today.
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