No tang stamp aside from "INOX".
It may be Swiss, not German. The handle has "Interlaken" imprinted on it. My father traveled for some months through Europe in the late 1930's. I had always assumed he acquired it during that time period, but I never asked him. I just remember that this was the knife he carried every day when I was growing up in the 50's and 60's.
Your knife is probably Swiss, a clue being the old patent pigtail corkscrew, which lasted until the 90's. But I'm unsure it is pre-WWII.
The can opener seems of a quite recent kind, different of those on period Swiss knives more like those on French knives.
INOX stands for inoxydable (stainless) used in France, Swiss, Spain and sometimes Italy. Stainless was used on SAKs from 1951.
The "Swiss" cross means not a lot, as makers in Thiers and others made such "swiss type" knives including cross (the Savoy cross is exactly the same, btw), generally more squared at the ends (but not always).
The fact most people call these knives SAK is a problem as they are
NOT the soldier's knife replica, but the Swiss Officer's (
Officier Suisse - some blade show it) replica.
An excellent site for Swiss soldier knife recognition :
http://www.couteaux-du-soldat-suisse.ch/J01/index.php/fr/les-couteaux-du-soldat I guess you can contact them.
You can see it is quite different of the officer's. :
http://www.victorinox.com/ch/content/development_SAK?lang=fr&
There were lots of swiss knives makers but most disappeared and there 's not much info except on Wenger and Victoria. This little tool is more distined to recent knives, but amusing to look at!
http://sakselector.info/cgi-bin/sak_selector.cgi