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US Army DUI Crests
Distinctive unit insignia (DUI) are authorized if the unit has at least 500 military personnel assigned .       

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.
   Each DUI Pin traced the traditions of that unit with symbols, colors or mottos.  Some units could trace their lineage back to the Civil War.  Some symbols represent service in foriegn campaigns, such as fish for the Philippine Insurection or a fleur-de-lis for service in France during WW1.  DUI Pins for the division was simply the division's shoulder patch represented on a pin.
   The DUI Pin was optional as many units did not seem to have one.  It was not worn in combat but only on the 4-pocket tunic or the "Ike" jacket.  A pair was worn on the lapels of the uniform and one could be worn on the overseas cap in place of the entlisted man's disk. 
   The pin used one of three methods of attachment.  The oldest method is the (a) screw-on pin which had one threaded stud and a large nut.  Some very old ones had (b) a safety pin clasp, simliar to a safety pin.  Most of the modern pins used (c) the "clutch-back pins" which used two pointed studs that had the modern spring-loaded caps.