This was the thinnest watch movement made from 1946.
Cal. 2003 was introduced in 1946 or 1953 as the thinnest watch movement available. It achieved this goal by passing the Piguet 21, introduced in 1925, by .1 mm: The F. Piguet movement was 1.75 mm and this Cal. 2003 was 1.64 mm. Both measured 9 ligne in diameter, making them extremely small as well. Cal. 2003 was designed in cooperation with LeCoultre and Vacheron Constantin.
The movement was based on the Audemars Piguet 9ML but with many modifications, specifically to the bridges. Audemars Piguet had exclusivity for two years before Vacheron Constantin introduced their own version, their Cal. 1003. Jaeger-LeCoultre never used the calibre themselves, but they did produce it as Cal. 803 for others to use.