When the marks are original the metal gets peened up around the stamp just slightly. When it gets refinished it gets sanded during preparation and that knocks that slight rise down flat, even though you can still see the stamp.
The wear line under the safety is a pretty easy to spot indicator that it has been refinished. There is no way that this gun made it from 1917 to now without wearing through the bluing between gesichert and not gesichert.
Here is a sticky on spotting refinishes in case you haven't seen it.
http://forum.lugerforum.com/showthread.php?t=17917
They are small differences but after you look at enough examples you get the idea. Better yet refinish a "shooter" shotgun or 22 and you will learn what it does first hand. It never hurts to ask if you have doubts. I would agree with Vlim that it has probably been touched up here and there, as some parts look ok, while others are a bit off.