The silver Purple Heart is from the initial requisition for 135,000 medals supplied by the U.S. Mint to the U.S. Navy in 1942. Due to the need to conserve metals such as copper and brass, this initial order was made from Sterling Silver and gold plated. They cost the Navy $2.56 each compared to about $1.76 each for medals made of bronze supplied by Rex Products and Robbins Co. to the U.S. Army. The silver medals were not numbered. Subsequent requisitions by the Navy made in 1944 and 1945 were made of bronze. Navy Purple Hearts made in 1945 were numbered. This information is to be found in "The Call of Duty: Military awards and decorations of the United States of America", by Strandberg and Bender;