Scotch and Soda with Brass Bang Ring by Johnson Precision Magic

Half Dollar and 20 Centavo piece are given to spectator to hold behind
his back. Magician asks for the half dollar which the spectator
successfully hands over. Next you ask for the 20 Centavo coin. When
the spectator opens his hand he finds that Mexican Coin has changed into
a quarter and he can examine both coins to his heart’s content. Again,
the only reason this is such a straight-forward miracle is due to the
precision machining of the coins. Here’s one you’ll really use.
Complete with bang ring. Easy to follow routine included.