Found this comment on eBay:
Occasionally in the world of antiques, you may come across a mark, DEP, which could mean either Depose or Deponiert. This mark is commonly found on German or French bisque dolls, celluloid, porcelain, jewelry, glass or metal goods. Dep stands for the German word Deponiert which means registered, or copyright. Dep is also a shortened form of Depose which is French for registered.Without knowing a manufacturer or country of origin, it is safe to say that if your item is marked DEP, it is most likely of French or German manufacture.