Over half his victories came from his time with Jagdstaffeln 72, until his reconnaissance mission above the Marne in which his machine was forced down he 'landed' in the tree canopy somewhere near Chateau Thierry or Castle Thierry as some call it.
I came across text supposedly from the Chicago Daily Tribune, 28 July 1918
written a United Press staff correspondent named Frank J. Taylor
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YANKEE AIRMAN IN FIRST FLIGHT GETS BOCHE 'ACE'
Lt. Avery Forces Capt. Medhopf, 16 Victories, Down
WITH THE AMERICANS IN FRANCE, July 27.-- Lieut. Walter Avery of Columbus, O., in his first air battle, brought down the noted German ace, Capt. Mendkopf, north of Chateau Thierry yesterday.
HUN TAKES FALL
The rival aviators came together while patrolling alone. For twenty minutes Mendkopf made futile effort to gain an advantage by tricky maneuvers. Then Avery got on the German's tail and damaged the plane so that it was forced to land within the American lines.
When Medkopf learned this was Avery's initial fight he was enraged. He sulked and refused to talk. Mendkopf is credited with sixteen victims, including an American. He is one of only six German aviators who wear the cross of merit.
MADE A MISTAKE
Another American airman, after hearing of Avery's exploit, went out hunting on his hook this morning. He tackled a Boche and forced him down and landed to complete the capture. But he discovered that he was behind the German lines and he himself was made a prisoner.
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This Avery victim seems Oberleutnant (not Captain)
Karl Menckhoff, brought down on the 25th. Why they thought he had 16 instead of 39 who knows - the press is usually off anyway, eh?
You may be able to chase archive newspapers, esp. in that area of France.
Or use Jasta 72 or C. Thierry as the focus of your search too. I don't know if M flew solo on that recon. Is there a good resource for Jasta 72 flight reports? I'm not sure M's plane wasn't so high stuck in the trees that it could really have been salvaged. Maybe pieces of it are still there...

I'll see if I can come upon anything else.