The other candidate is from a lot longer
link ago. This was the Jaeger-LeCoultre caliber 145 which at 1.38mm is thinner than any
link version of the caliber 849 and which was made for a surprisingly long time from about 1907 all the way up to the 1960s. This is a pocket watch movement okay but it apparently was reliable in the JLC movement catalog for over fifty
link years and thinner than any current-production movement of any kind complicated or simple hand-wound or automatic.