runner -
Stock lugs are not found on the German Army 4" Lugers prior to late 1913. The directive for adding stock lugs was issued in late 1913, and many 1913 Lugers were manufactured with such a lug. Of note, a few of the 1914 DWM Lugers were manufactured without stock lugs.
Another suggestion which has been offered in earlier posts postulates that manufacturing efficiency was improved by machining all Luger frames, not just Navys and Artillerys, to a uniform pattern as opposed to the production of two different varieties.
Luke