Howdy Mojjax, it's been a while! I'm not an expert on these, but took a moment for a fellow old timer, and found possibly some information: From what I find online this mark is often accompanied by an incision by Rose O'Neill. I can see as much on yours as I can on the others of such a mark, which is of course inconclusive, but may help. Further, I only saw this mark on earlier ones (1912-1920ish) and only on German ones. Hope this helps. Be well!