This Jaeger- LeCoultre '101' wristwatch movement introduced in 1929 remains the smallest mechanical wristwatch movement ever made.
It was put into series production and remained available at least until the 1950s.
Queen Elizabeth II got one of this watch in 1950's.
It comprises 98 parts, measures 14mm x 4.8mm x 3.4mm, and weighs only 0.9 grammes (including the dial and hands). LeCoultre company have manufactured precision watch components since 1833.
In 1903 Jacques David Le Coultre entered into a partnership with the chronometer-maker Edmond Jaeger and together they manufactured some of the most technologically advanced watches of the era. They also introduced the Reverso watch in 1931, which became one of the best known watches in the world.