This is a nesting set of three shells and one half dollar.  It allows for many possible effects.  For example...

The magician shows four half dollars in his hand. He takes a coin and puts it in his pocket. He shows the hand and has four coins again. He takes a coin once again and puts it in his pocket, but he has four coins again in his hand. He repeats this operation once again and he always has four coins in his hand.

Finally, he show four coins in his hand, takes a coin and puts in his pocket, opens the hand and the rest of the coins have disappeared!