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It says "American", but the men's uniform look more appropriate for British troops. If I recall correctly, the US uniform featured flat, not pleated pockets and a stand-up collar.
Well there goes the "accurate details of uniform " blurb.
The driver could be from one of the various volunteer services so is not necessarily a military officers. The lace up linesman boots were not uncommon.
A lot of ambulances were operated by volunteers and serviced all of the combatants. The wounded actually look generic as those may be under shirts (no bottom pocket or flares and pleats in the back generally found in period tunics) If so they could be French, Italian, Serbian, Greek. Volunteer ambulance services were on other fronts so you're not stuck with US no matter what the label says.