I was wondering if there might be a hallmark under the photo or the glued-in piece of paper, but I'm not sure I'd be brave enough to investigate. Alternatively, it's not pewter, is it? In which case there wouldn't be a hallmark. My feeling is that the royal arms mark it out as an English (or British) piece, and that Amiens has been engraved later, in a rather less formal style than the arms suggest.
As for its origins, it may be relevant that during WW1 there were thousands of British civilians in northern France doing volunteer work. There were nurses looking after the injured, and others who worked with the YMCA and other organisations in huts providing rest, recreation and refreshments for off-duty troops on their way to or from the front line. I wonder if maybe the locket could be evidence of a romance involving a lady volunteer, in or near Amiens.
Many of the volunteers' names can now be found on the Lives of the First World War website, and some on medal rolls etc too, but until fairly recently, much of the evidence for their involvement has been in their own family stories, letters and papers etc. If you have any possible names for ladies who could have served in this way, it might be interesting to search for them on the Lives website to see what comes up.
Arthur
