US Army DUI Crests
71st AFA example.
Distinguishing Unit Insignias were worn on the uniform to identify the basic unit, such as an infantry regiment , medical or artillery battalion, as an example.These were made of either brass (stopped in 1943 to save material for ammunition) or sterling silver (after 1943) and usually in the shape of a shield. Some were coated with enamel or painted. After the units went to Europe, new pins were needed for the replacement troops when they arrived. This resulted in the many pins that were made locally in Germany and Italy.
Most of above taken from: http://www.custermen.com/Reference/Uniforms/Insignia.html#DUI_Pins