[carguychris]

I'm not familiar with shoulder-stock Lugers specifically, but other pistols set up for shoulder stocks typically have a vertical slot or tabs on the lower rear grip frame where the stock would attach.

As I said earlier, AFAIK the vast majority of Lugers were not outfitted for shoulder-stock use, and IIRC most of these date from the WWI era, although I'm no expert on these pistols.