The story behind the Patek Philippe 5100 is that Patek has always had a tendency to make commemorative watches to mark important anniversaries and milestones, like their own 100 year birthday, or long standing important partnerships like the special edition watches for Wempe and Tiffany. The reference 5100 was made to commemorate the turn of the century in 2000. Given the important occasion, they brought out the first 10 day power reserve movement ever created by any manufacturer, ever, in a wristwatch. Further, Patek used this unique case shape, which was inspired by the manta ray.
The case is 18k gold with dimensions of 46mm long by 34mm wide. Front and back crystals are sapphire. The case finish is polished. The crown is protected by the shoulders surrounding it.
Movement is the manual winding Patek Philippe Caliber 28-20 with 29 jewels, many of which are in gold chatons, free sprung Gyromax balance which is adjusted to heat, cold, isochronism and in 5 positions, and is stamped with the Geneva Seal. The 10 days of power reserve are provided by twin barrels.