The thinnest mechanical watch ever made. The Bulgari Octo Finissimo Ultra measures about 0.067 in (1.7 mm) thick, making it the thinnest mechanical watch on record. Bulgari achieved this by integrating the movement directly into the caseback, removing the usual base plate.

https://www.reddit.com/r/Damnthatsinteresting/comments/1poefgy/the_thinnest_mechanical_watch_ever_made_the/

Only 20 made, all yours for a mere US$529,000.