Cal. 51 is a simple cam-switching chronograph movement produced by Landeron in the 1950s. It was part of a larger family of movements based on Cal. 48 and is closely related to calibers 55 and 151, 159 (with date pointer), 57, 59, 80, and 180 (with full calendar), and 81 and 181 (with full calendar and moon phase indicator).
Production is said to have begun in 1947, and the movement was updated around 1950 with a new hammer and cam mechanism. Unlike many movements in the family it uses a traditional horizontal clutch to activate the chronograph function.
