The ballast story is correct, but my understanding is that it was more a metal arbitrage game. The Merchant on board the ship had instructions to sell copper wherever the price was above what the king of Sweden wanted (the king was the owner of the great Swedish copper mines), so that surplus copper would not disturb the copper market price. In other words, the plate "coins" were treated as a commodity, not as money. Today's equivalent would be Kruger rands (subject to VAT), not US quarters in Zimbabwe.
@ mrbadexample: I can recommend a visit to Sweden. The royal coin cabinet in Stockholm has an impressive collection of plate coins.