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The aviation-inspired case and engine of the Thunderbolt are one. Neither would, nor could, exist without the other, yet each is so transcendental as to be able to stand alone as a work of art in its own right. The case of HM4 blends high-tech titanium for its lightweight and strength with a sapphire centre section offering a view into the engine. The limited editions HM4 Razzle Dazzle and Double Trouble take the aviation theme even further with real rivets in their fuselages and hand painted nose art inspired by the rebellious paintings on WWII aircraft, while HM4 RT offers an altogether more luxurious way to fly. The Thunderbolt's engine is the culmination of three long years of development. Each of the 300-plus components – including the regulator and even the screws – was developed specifically for this anarchistic calibre. Horizontally configured dual mainspring barrels drive two vertical gear trains, transferring power to the twin pods indicating hours/minutes and power reserve. The sleek aerodynamic form of the Thunderbolt's envelope has its roots in Maximilian Büsser's childhood passion for assembling model plane kits, though none looked remotely as futuristic as these. The striking transparent sapphire section of the case requires over 185 hours of machining and polishing to transform an opaque solid block of crystal into a complex, exquisitely curved panel allowing the light to come in and the beauty of Thunderbolt's engine to stand out. Every component and form has a technical purpose; nothing is superfluous and every line and curve is in poetic harmony. Articulated lugs ensure supreme comfort. Highly legible time is a fringe benefit. HM4 is available in titanium/sapphire; HM4 Razzle Dazzle and Double Trouble are limited editions of 8 pieces each in titanium/sapphire with hand painted nose art; HM4 RT is a limited edition of 18 pieces in 5N red gold, titanium and sapphire and the HM4 Final Edition closes the HM4 series in 2013 with a limited edition of 8 pieces in blackened titanium and sapphire. The present model is a piece unique made in yellow gold. Dimensions: 54mm wide x 52mm long x 24mm high