...the Tourbillon Bi-Cylindrique with Heures Mystérieuses ! The Heures Mystérieuses are based upon the principle of two sapphir discs rotating one above the other. It comes from the glory days of Cartier at the beginning of the century. The bi-cylindrique tourbillon is an invention from Montblanc.
As a reminder, the mechanism is a cylindrically shaped balance-spring: unlike the vast majority of contemporary balance-springs, this one doesn't swing to and from in a single plane, but expands and contracts along a vertical axis as though its gyres were the threads around a bolt.
Don't know which one is the most mesmerizing....!!