I will post a better photograph of the pistol later today but it can be seen in this photograph. The fellow in the middle is my father (I was adopted rather late in his life) It looks as though I was mistaken about the plane being Spad but it's clear that they have . . . ahem . . . liberated a few souvenirs. There is no crown on the pistol nor is it ornate which makes me wonder if perhaps it was perhaps an assasen's weapon, used to shoot someone of some significance. I can think of no other reason for a common gun to have made it into the Kaiser's collection. The photo was taken around 1918.
Look at the other interesting artifacts in the picture too. The short barreled rifle on the left and the cane (gun?) on the right.
Sadly, he passed away when I was two years old so I never got to hear the story first hand.