The patent safe game was a confidence trick much practiced during the late Civil War in and around New York City. It is a variation on the coin-matching game, in which the person to be scammed believes they are party to a trick only they and a second person know.
It was carried on principally in the neighborhood of the Hudson River Depot, and the complaints of the victims, to the police, were loud and numerous.
A contemporary account in The Secrets of the Great City describes the trick:
A stranger in the city would be accosted by a well-dressed individual, who would immediately begin a careless, friendly conversation. If the overtures of this individual are not repulsed in the first instance, he is soon joined by his accomplice, who professes to be a stranger to swindler number one.
The accomplice has in his possession a small brass ball or sphere, which he says is the model of a patent safe, much used by merchants in China and India. He is trying to introduce it in this country, and would like to show the gentleman his model. This brass ball is, to all appearance, solid, but to the initiated it is soon made hollow, by pressing on a certain inner circle, when the centre of the ball, which is in the shape of a small cone, drops out. The bottom of the cone may be unscrewed, when a little chamber is revealed, in which is a long piece of white paper, carefully folded and secreted. The other end of the cone, the top of it, can be unscrewed, and a second chamber is revealed, in which is a second piece of paper, exactly like the first.