The USA National Defense Service Medal (NDSM) initially used small bronze stars devices to indicate multiple qualifications.  The bronze star devices were the same size for the ribbon bar and the full size medal ribbon.  A miniature bronze star (about 1/2 the size) was worn on the miniature medal ribbon.  While some of the bronze star devices were official Government Issue, the vast majority were purchased by the recipients and were from His Lordship Industries, Susco, Vanguard, Ira Green, NS Meyer, and many more smaller makers.  There are no hallmarks to distinguish them but many collectors can readily identify the majority.  Normally a small silver star device replaces 5 bronze stars devices but the NDSM hasn't been authorized for 5 periods as of this date.   During the Vietnam War era, the 1st device was authorized and many individual unit personnel offices stocked a small supply of these devices for qualified servicemembers.  By about 1974-1980, only the larger military installations issued the first device.

The US Army has changed the regulations a few times specifying that a bronze oak leaf cluster is to be used in lieu of the bronze stars device.  These continual changes resulted in confusion and it is very difficult to determine the era represented since both systems were often used concurrently.  When researching US Army NDSMs with a device, the best suggestion is to view as many contemporary photos as possible.  

It's also interesting to note that recipients of the NDSM normally qualified for another ribbon or medal so a verified recipient of the NDSM with a device is probably impossible.   Devices were not presented to the servicemember (although a few small units did attempt to try this tradition) and there was no certificate or document of any type.  All documentation was in the personnel records which servicemembers often had copies of  but the official documentation remains the DD-214 document.   

For further information, please see my blog (http://blogs.ebay.com/tsa-li).  Extensive information is also available online.