Shoulder stocks were not issued with standard handguns, only the Artillery Models... The use of any stock on a Luger handgun that is not on the ATF Curio & Relic List of exempted NFA stocked pistols would be a federal felony...under the National Firearms Act of 1934, UNLESS you register the gun/stock combination as a Short Barreled Rifle (SBR) with the ATF, and pay the $200.00 tax.
Here is a photo of a stock (without holster and accessories) typical of the artillery type:
Here is a small photo of this type of stock attached to an Artillery Luger with a TM-08 (trommelmagazine) snail drum 32 round magazine that we use on the front page of this website.