More research is needed, but "duck hunter" patterns were used in Vietnam, supplied by the CIA from American civilian sources. Update: this pattern is closely related to the Vietnamese-made example of “beo gam” (leopard) on camopedia.org (turn the sample 90 degrees counter-clockwise, and compare to the back of the shirt).
Ex-Peter Coleman collection. Pictured in "Wee Warriors and Playtime Patriots", p.201.