Bob, why do you think "it's the Cav"? The insignia on the door is not a 1st Cavalry patch.
Here's a thought. Anyone besides me think it might be a photo from a training exercise some where like Louisiana long before deployment circa 1964-65?
Reasoning: There is no magazine in the door gunner's rifle. The helicopters are lined up nice and neat on the paddy dikes. The helicopters have nose numbers similar to what one sees in flight training establishments.