Also known as a unit crest or DUI, a Distinctive Unit Insignia is worn by all Soldiers (except General Officers) in units that have been authorized to be issued the device. It is worn centered on the shoulder loops of the Army Green Service Uniform (AGSU) and the blue Army Service Uniform (ASU, Enlisted only) with the base of the insignia toward the outside shoulder seam. DUIs are not worn on the Dress variations of either uniform, however.
The 517th Air Defense Artillery Regiment Distinctive Unit Insignia was approved on 29 December 1958. It had previously been approved for the 517th Coast Artillery (Antiaircraft) Regiment (April 1938 to October 1940), the 79th Coast Artillery (AA) Regiment (October 1940 to June 1949), the 78th AAA Gun Battery (June 1949 to June 1956), and the 78th Antiaircraft Artillery Missile Battalion (June 1956 to December 1958).
Scarlet has been either the sole or primary branch color for Artillery units—including Air Defense Artillery units, even though they are in a distinct branch—since the early 1800s, but the images of a dive bomber in what appears to be a spotlight makes it clear that this is an Air Defense Artillery insignia rather than standard artillery; if not, the motto “We Sweep The Sky” should clear up any confusion.
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The 517th Air Defense Artillery was originally constituted in the Organized Reserves as the 517th Coast Artillery in 1923. It would undergo seemingly countless redesignations and reorganizations over the next five decades; on the organizational level, the most notable of these came in 1957 when it became the 517th Artillery and in 1971 when it was redesignated as an Air Defense Artillery Regiment, both parent organizations in the Combat Arms Regimental System. The Regiments and its six Battalions were credited with participation in the Aleutian Islands campaign in World War II and were all awarded Arrowhead devices for taking part in an assault landing; they also took part in all ten of the named campaigns of the Korean War.
It is not clear when the last active Battalion was inactivated, but it would presumably be no later than in the mid-1990s following the fall of the Soviet Union and the subsequently reduced threat of missile attacks.