Calibre 838 and similar calibre 839 is an ultra-thin mechanical movement from Jaeger-LeCoultre.
Introduced in 1962, calibre 838/839 measures 20.8 mm in diameter and just 1.84 mm thick. It has 18 jewels and a 36 hour power reserve.
In addition to Jaeger-LeCoultre, Chopard and IWC also used this movement.