Few connoisseurs are aware of the extent to which the interior architecture of the Royal Oak's case has been modified during its history for various reasons – sometimes difficult to pinpoint. These modifications stem from the need to correct defects, but above all from the desire to improve performance and find new technical solutions. Above all, they express the fertile imagination of case construction engineers.

The first major change occurred in 1976 and 1977, no doubt at the request of watchmakers. Models 8638 and 4100 gave up the monocoque case and adopted the classic "three-part" structure, enabling easier access to the movement for touch-ups. The water-resistance system was also revised at the same time and the huge black seal was replaced by small flat traditional ones. To fill the space, the bezel was redesigned and extended beneath its visible part: its thickness was increased to 5.3 mm in Model 4100! This system brought rigidity to the case and saved space, thus enabling the Royal Oak 4100 (35 mm) to gain an additional 4 mm in diameter compared to Model 5402 (39 mm), even though it houses a movement that is only 2 mm smaller than the latter.

Nevertheless, this change did not solve all the problems. In 1983, Model 4332 (36 mm) returned to the monocoque and revived the oversized seal, of which the quality had been improved. Incidentally, some seals in the 1980s were transparent, e.g. that of Model 14486. In 1990, Model 14700 retained the monocoque structure but significantly reduced the width of the seal, while increasing the width of the casing ring. Two years later, Model 14790 reintroduced proportions close to those of the original.

In 1984, another solution was invented. The first Royal Oak Perpetual Calendar, Model 5554, replaced the oversized rubber seal with a very large metal casing ring, independent of the bezel. The idea was quickly abandoned as it jeopardised the watch’s water-resistance (an optimised version of this system was adopted 28 years later on Model 15400). The Customer Service department then created a procedure to correct the 5554 casing rings. At the same time, Audemars Piguet introduced the Royal Oak 5654, aesthetically identical to the 5554, but with a radically different case structure. The solution chosen was different from all the previous ones: the casing ring was the same width as the seal and the screws passed between these two components in half cylinders. It was retained on Model 25636 watches (and their derivatives) from 1993.

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