I have just been informed by the Facebook Group WW1 Buffs that The Western Front Association Thames Valley Branch has details of the 'Captain S" mentioned in Mildred Aldrich's "Hilltop on the Marne", including a photo of 'Captain S":
There is also a suggestion that the person Mildred corresponded with in America during the early days of the War (her letters form the bulk of the book) is Gertrude Stein, the American poet who lived in Paris.
However, I thought Gertrude purchased an ambulance during the First World War when she lived in Paris and drove it to take supplies to hospitals in the area. I think a little more research is needed...
with grateful thanks to the wonderful Facebook Group WW1 Buffs - find them here: https://www.facebook.com/pages/WW1-Buffs/162842593777796?fref=ts
How very sad. I expect Mildred was devastated.