Patek Philippe CH 28-520 C

The caliber of the Nautilus 5980. Two features identify it and both are visible from the dial side: a single 60-minute counter rather than the conventional 30-minute counter, and the flyback-free vertical-clutch behaviour of the chronograph seconds, which starts without the stutter a horizontal clutch produces. The 60-minute counter in particular is unusual enough that a Nautilus chronograph dial with a 30-minute register is a question about the dial.