Hey Mark,
I believe that he could have run the chin strap through the slits, but if we knew that reason why he reversed the Überzeug, then we would have a better idea about the chin strap. Did he reverse the Überzeug because he was not allowed to have a photo taken with his unit number visible? If so, that raises the question of why he did not just remove the Überzeug, would a front line soldier prefer to have his photo taken with an article of uniform that would denote he is one of the men fighting the war? This is the kind of photo that opens up the mind of the subject to give us a chance to speculate what he might be thinking, a rare photo indeed.
Gus
PS Maggie says that it was just a new recrute who did not know haw to put it on he he he rsrsrs