Hate to burst your bubble, but there is no stock which can be legally attached to your Luger. Only certain stocks can be attached to LP-08s (Artillery models), Navy models, and 1900-1902-1906 American Eagle models with particular grips.
http://www.lugerforum.com/BATF.html
http://www.lugerforum.com/BATF2.html
are in the information portion of this Lugerforum. They are the pertinent parts of the BATF regulations concerning Lugers and shoulder stocks.
--Dwight