Jos,
I read through Volck's book "Die Woelfe" (the wolfs) again.
He did not specify any dates or units. Maybe because the war was still going on. In the foreword he mentions that he gave an oral presentation to Fieldmarshall von Hindenburg on 02/23/1918, and Hindenburg wished he would write a book about his experience.
Here a picture, Herbert Volk is bottom right:
The description reads: "Herbert Volck and his escape companion Josef Reis(bottom left) after their 1000km foot march through Persia at the the German Flying Headquarter in Mossul, January 1918"