Secret compartment cufflinks

Saw these for 1,900 nickels on Fancy.  Very cold war.

After seeing these nickel cufflinks I googled around for other functional cufflinks and found the following (a lot of them can be found on a UK based watch part company):

Movable time and date

Working sand timer

Dice! 

Called the Square ball game.  The balls move but I'm not sure this makes it a game. 

This one however, is a real puzzle.

Levels in cool blue.

Thermometers

Lighters

Pocket knives... really small ones.