Thin watch movements were all the rage in the 1950's, with the 1.75 mm Piguet 21 being displaced by the Audemars Piguet/Jaeger-LeCoultre/Vacheron Constantin Cal. 2003 in 1953. Piaget developed their own competing movement, introducing Cal. 9P in 1957. At 2 mm thick, it wasn't quite competitive with these others, but it became famous nonetheless.
Cal. 9P remains in production today.