A series of low-level photographs clearly showing a set of four digit numbers, followed by the probable letters "TH" or "TA" was first imaged on May 20, 1973, in north central Laos. The site was apparently imaged again, with the numbers and letters still visible, as late as 10 July 1973. DIA did not receive the film, taken by a low-level reconnaissance platform, until 1976.
According to the ASD C3I, two sets of symbols are clearly man-made. The first is the symbol, 1973 TH, taken on July 10, 1973, on the Plain of Jars in Laos. Some interpreters believe that the "TH" could be a "TA" and the 1973 could be "1573." DIA attempted to correlate the four different interpretations (1973 TH, 1973 TA, 1573 TH, 1573 TA) to classic distress symbols, escape and evasion symbols, or personal authenticator numbers. Although there was no exact correlation, the ASD C3I offered several alternatives as possible explanations for the ground symbols. These included: markings made by the crew of a CIA-operated aircraft downed eight kilometers from the site on May 7, 1973; symbols made by Thai personnel captured in the area by Pathet Lao forces; markings made by members of the crew of an U.S. AC-130 gunship downed 300 miles away in southern Laos in December 1972. DIA believes that none of the alternatives are definitive and has concluded that "the origin and meaning of this symbol is unexplained and probably will remain so." [Senate Select POW-MIA Affairs Report]