Cal. 9ML (also known as Cal. 9”’ML) was an ultra-thin hand-winding movement from Audemars Piguet. It was the basis for Cal. 2003, which would remain in production for half a century as the thinnest movement available.
There is some controversy as to the date of introduction of Cal. 9ML, with some sources claiming it was produced as early as 1938 in limited numbers and others (including a timeline on the Audemars Piguet website in the 2000s) stating 1946. The latter date is also sometimes claimed for Cal. 2003, which was not definitively known to be in production until 1953. Since these movements share many characteristics, including the groundbreaking 1.64 mm ultra-slim height, it is understandable that they would be confused.
According to one source (linked below), just 772 examples of Cal. 9ML were produced over 15 years of availability. This includes an ultra-thin wristwatch and even a coin watch.
Cal. 9ML featured individual bridges and plates for most bearings, in classic pocket watch style. This was refined for Cal. 2003 to just two plates for added durability.