A. Lange & Söhne Lange 31
The Lange 31 by A. Lange & Söhne is the world's first mechanical wristwatch with a power reserve of 31 days and patented constant-force escapement. With the aid of which it is now possible to use a high power reserve purposely. Because during the entire term this mechanism ensures the highest rate stability and accuracy.
For a wristwatch a key wind is very unusual. But compared to a crown winding it allows to choose a significantly larger winding translation. So less winding revolutions are necessary - the energy can be built faster and more convenient.
The key is set in a recess in caseback with sapphire crystal. Three seals between the winding mechanism and case protect the watch against a pressure of 3 bar. In order to prevent inadvertent pulling of the springs, a mechanism for torque limit has been integrated. If you still try to wind when the movement is fully wound, the triangular teeth of two discs dissolves inside the key, and the power flow is interrupted.
The Lange 31 is only available with platinum case.