In another picture, a New Zealand officer, smoking a pipe, is seated beside her. His identity is also unknown, although a New Zealand military analyst said he was a lieutenant in the 7th Southland Mounted Rifles.
Other photos show him standing beside her, and posing with a group of Kiwi cavalrymen.
The pictures have mystified Dominique Zanardi, a cafe owner in the Somme village of Pozieres who runs a museum of wartime memorabilia. "It's unusual for a woman to have been an officer then," he said.
He had previously seen photographs of French locals dressing up in uniform, "but the sleeves are too long for them, and something seems wrong, whereas this uniform fits the woman perfectly".
New Zealand historian Andrew Macdonald said that "clearly, the woman is wearing the officer's spare tunic". But were they married, or was the mystery woman the French mistress of the Kiwi officer? The enigma remains.
Mr Zanardi received the plates - in a battered wartime cardboard box containing 25 negative plates - from a contact.
The other pictures show British officers and a South African in similar poses, occasionally sitting with their hosts.
"In the rear bases, the officers stayed with the locals. There must have been two million soldiers at the height of the war, and they needed to stay somewhere," said Mr Zanardi.
Photographer Bernard Gardin, who transferred the pictures onto digital media, said several plates were damaged by fungus. "They are very fragile, and hate humidity," he said.
He is transferring another pack of about 40 plates which were found in the village of Guignemicourt, not far from Hallencourt.
Almost 100 years after the beginning of the 1914-1918 war, the Somme continues to yield its secrets. Soldiers' bodies are occasionally dug up in the fields, and Mr Zanardi says local people are still coming forward with memorabilia from barns and attics. The latest pack of 40 plates was discovered by a farmer after he bought a house, and last year, 400 glass negatives were found on a rubbish heap.
The photographs of the Kiwi officers date from 1916, when 15,000 New Zealand soldiers were deployed in the Somme.
Mr Zanardi noted that the New Zealand military moved their insignia on to epaulettes in 1917 when their uniforms were revised. Earlier in the war German snipers had targeted the Commonwealth officers, whose rank was clearly visible.
The New Zealand Division went into action on September 15, 1916, in support of a major push in which British tanks were used for the first time to seize the village of Flers, near Longueval.
By the time the division withdrew in October, more than 2,000 New Zealanders were dead.
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