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;

You will find the most comprehensive description of US awards and their "Attachments and Appurtenances" in "Military Medals of the United States" by Colonel Frank Foster and Lawrence Borts. US medal design is controlled by the Institute of Heraldry (managed on behalf of all four services by the US Army) and the Commission of Fine Arts. Your question is a logical one, but the US award system allows for a wide variety of ribbon emblems to be worn on each award. The meaning of these varies and there can be emblems on a single ribbon denoting multiple awards, acts of valour or merit, or multiple tours of duty etc. British and Commonwealth awards only permit attachments that show one additional type of service; e.g. multiple awards, or same service in another location, or periods of additional long service, or service in a specific combat zone (Atlantic Medal). The relatively short length of ribbon on US awards limits space available and precludes the use of "Bars" common to British and Commonwealth awards. I have 'Court Mounted' numerous US awards on medal bars for New Zealand service personnel, both full-size and miniature, and it is a difficult job.

I would agree, if only looking at the reverse. But the Zimmerman obverse is by far my favourite.