Why is a 1970s American soldier in a 1914 World War I photograph?

https://upload.wikimedia.org/wikipedia/commons/2/2b/4th_Dragoon_Guards_at_Mons_1914.jpg

Five British soldiers on the right are in 1914 uniforms, more suited for parades and field exercises in peacetime than in a full-scale war. Uniforms changed as the battles in Europe developed into four years of trench warfare.

The soldier on the lower left, though, is from another army, as shown by his uniform. He wears the U.S. M1 helmet, which was issued from 1942-85. What marks the soldier as being from the 1970s, he is wearing what was commonly called a “flak vest,” capable of sometimes stopping had grenade and mortar fragments. The soldier also appears to wear a jungle fatigue type shirt and a pistol belt.

The Battle of Mons, Aug. 23, 1914, was the first between the British army and invading Imperial Germany forces. With the presence of 1970s American soldiers, maybe the British had more help than historians know.