This is a little complicated: A client of mine had a very unusual timepiece, a $20 gold coin that was milled out and had a gold watch movement inside it. This was made by Audemars Piguet, a very fine Swiss watchmaking company. It had a secret latch and hinge that sprung open to reveal the watch inside, when closed, it appeared to be a simple gold coin.
This is where it got complicated, my client wanted to wear this coin as a wristwatch. This meant that a frame had to be made to hold the coin, and it would need another latch to activate the secret latch. The coin must also be removable from the frame.
The buckle for the watch was also hand made from 18Kt yellow and rose colored gold. This style of buckle is known as a "deployment" buckle.
As you can see, when assembled, the face of the coin flipped open to reveal the watch inside.