The formation of the Royal Australian Navy Helicopter Flight Vietnam was announced in July 1967 to be integrated with the United States Army 135th Assault Helicopter Company (AHC). Based in Vung Tau, the initial flight consisted of eight airmen flying Iroquois helicopters, and their requisite staff.
On January 12, 1968, just four days after the postmark date, the cover's recipient, Keith Wardle, was injured when his gunship was forced down in the jungle. The wreck caught fire, detonating ammunition and rockets. Wardle was seriously wounded and evacuated to Australia.
RAN pilots ceased flying June 8, 1971 as the United States was disengaging from Vietnam.
Details about the RAN Navy Helicopter flight may be found on this excellent site: http://www.navy.gov.au/RAN_Helicopter_Flight_Vietnam