I have now had a good look at the material in Arthur's field letter case (the tin box) and it is quite a trove. His MC is there along with other awards and a ton of letters, photographs and papers. Some of his personal effects are quite amazing. For instance, a tubular silver case for just one cigarette with a lighter in one end and a compass at the other. One for smoking in a field whilst waiting to be picked up by the squadron Crossley Tender after the engine stops... or worse. The sense of immediacy is amazing, it's like breathing in pure history - a snapshot of someones life. As Piers and I rooted carefully through the contents we were simply bowled over by the story, most of it yet to unfold.
Note: The Spad above was part of a lineup shown in other pictures in the box. Either the British squadron was co-located, they were visitors, or perhaps Arthur grabbed his camera and popped across for a drink with the French/Americans. I don't think the RFC/RAF operated the aircraft; it shouldn't be difficult to place such a distinctively marked aircraft squadron.