The Audemars Piguet Royal Oak Reference 4100, also known as the Royal Oak III, holds a significant place in the collection, introduced in 1977 as the successor to the 5402 and 8638 models.

This pioneering timepiece marked the first Royal Oak to feature a center seconds hand, aiming to cater to those who found the 5402 too large, particularly in regions like Italy, the United States, and Asia. With a reduced diameter of 35 mm, the 4100 set new standards and was available in steel, yellow gold, and two-tone versions. Most models housed Calibre 2123, though later variations featured Calibre 2125, and some adopted Calibre 2124.

Production spanned from 1977 to 1985, with over 8,400 units crafted. The heart of the 4100, Calibre 2123, debuted in 1977, meeting the watch's specific requirements. Despite its smaller size, the caliber maintained a high frequency of 28,800 vph, allowing for the inclusion of a center seconds hand. Just under 9,000 units of Calibre 2123 were produced between 1977 and 1984, with notable improvements in 1979, giving rise to the 2123/1 variation.