Hi Rod,
Is sure would be nice to find a WWI-era photo with a stocked 4" Luger in a German soldier's hands.
If "necessity if the mother of invention", I can easily see a soldier trying a wooden stock on a short-barrelled Luger especially if others tell him how it has improved their shooting accuracy with a 6" or 8" Luger with a stock. But it would be nice to see such a period-photo for any "proof".
The SA museum folks will give you info, (if you email them) to see if the gun and stock came into their collection as a set or whether the "assemblage" resulted when the curator wanted to set up the display. Their acquistion of the gun and stock may have just come to them that way from a collector in the 1920's that decided to buy the pair together (and when it was not illegal in the USA to do so...).
Photo Rod mentions...