Google
Piaget was the undeniable champion in this field with the release of Caliber 9P in 1957 – the thinnest manually wound movement, only 2 mm in height – and the Caliber 12P in 1960, the thinnest automatic movement, only 2.3 mm in height.

https://www.hodinkee.com/articles/ultra-thin-what-it-is-why-it-matters-and-who-does-it-best-part-i

Piaget 9P
Type: Hand-wound mechanical
Shape: Round
Functions: Minutes, Hours
Power reserve (in hours): Approx. 36
Frequency (vph): 19,800
Frequency (Hz): 2.75
Diameter (lines): 9.00
Diameter (mm): 20.5
Number of jewels: 18
Thickness (mm): 2
Number of components: 89
Finishing: Straight Côtes de Genève, Circular-grained mainplate, Blued screws, Bridges circular-grained and bevelled by hand

Piaget 9P ultra-thin hand-wound mechnical movement
Image 1 - PIAGET 9P Vintage Watch Movement Balance is Ok (2501)


Manufacture Piaget 9P ultra-thin hand-wound mechanical movement. The legendary 9P movement, launched in 1957, is an ultra-thin hand-wound mechanical movement, measuring only 2 mm thick. Simple, sturdy and ultra-thin, the movement features central hour and minute functions.
https://www.piaget.com/ww-en/movement/9p-ultra-thin-mechanical-movement
https://quillandpad.com/2016/07/11/piaget-12p-birth-ultra-thin-legacy/
https://watchbase.com/piaget/caliber/9p