I get the feeling that a big slice of that remaining 1000 still wouldn't be identified even if they were genetically sampling at birth. Simply because the kind of person who ends up as an unidentified body in the U.S. is less likely to start out being born in a hospital, or being born in the U.S. at all.
I was 20 at the time and serving in the army. There I was actually leading company-strength motor marches.
Talk about overly worried parents.
As to your question, a quick Google arrives at a story claiming the number is 4,400:
"U.S. medical examiner and coroners' offices receive an estimated 4,400 unidentified human bodies every year, according to the first national census of medical-legal death investigations, "Medical Examiners and Coroners' Offices, 2004." Of these, about 1,000 are still unidentified after one year, and 600 are buried or cremated."
http://www.sixwise.com/newsletters/07/07/11/unidentified-hum...