Thanks for those very helpful replies. I have also dug something up since posting my question a few days ago. A site called The War Store has a listing for a 5SAI helmet flash as worn in the German East African campaign. It is mounted on a leather backing with a safety pin, so it would have been easy to reverse the alignment of the colours depending on which side of the helmet it was placed. It is also clearly green.
5SAI_Flash_front.jpg
5SAI_Flash_back.jpg