; having three layers with one layer containing 87 percent or more but not over 89 percent by weight of silver and 11 percent or more but not over 3 percent of tin, a second layer containing 99.9 percent or more by weight of silver, and a third layer containing 14.5 percent or more but not over 15.5 percent by weight of silver, 79 percent or more but not over 81 percent of copper and 4.8 percent or more but not over 5.2 percent of phosphorus