The Coin Safe is a nice looking brass container which has a lid with a hole in it for viewing the contents inside. The Magician shows the spectators an empty container. The spectators can inspect the container and view inside to see that it is empty.
A coin is then produced by the magician or borrowed from a spectator and the spectator initials the coin with a marker. The coin and container are then placed behind the magician's back and within seconds the coin is shown to now be inside the conatiner which is still closed and inpenetrable.
Through the hole, the spectators can see the marked coin locked inside. The spectators may again freely inspect the container until satisfied that there is no way to get the coin out of the container without breaking it. The magician then places the container behind hes back and removes the marked coin from the container.
 
All can then be handed out for inspection.