(set in 1917, written in 1920, published in 1925)
As the men grew more frightened at this undersea imprisonment, some of them began to mutter again about Lieut. Klenze’s ivory image, but the sight of an automatic pistol calmed them … I shot all six men, for it was necessary, and made sure that none remained alive.
The issue pistol of the Kaiserliche Marine in 1917 was indeed an “automatic pistol,” specifically the DWM-Luger P.04 (Investigator Weapons 1: The 1920s and 1930s, pp. 45-46) in 9×19mm Parabellum. Compared to the better-known DWM-Luger P.08, this has a longer 15-cm (6”) barrel and, at least in the early production models, a grip safety. Although generally issued with a detachable wooden shoulder stock and thus capable of being used as a pistol-carbine topside, navy officers usually wore it in a closed belt holster without the stock.
Officers like Korvettenkapitän Karl Heinrich Graf von Altberg-Ehrenstein were permitted to carry their own, privately-purchased sidearm, and many in fact chose smaller and lighter pocket pistols, which would have been better suited to use inside the confines of an U-Boot like U 29. The pistol in question might therefore have been another weapon entirely, such as an FN-Browning Mle 1900 (Investigator Weapons 1, p. 47) or Mauser Selbstlade-Pistole 7,65mm (C14), both in .32 ACP (7.65×17mmSR Browning). The nationalistic von Altberg-Ehrenstein would undoubtedly have preferred a German pattern over an American-designed, Belgian-made weapon.
Given the carnage wrought by von Altberg-Ehrenstein, the more powerful P.04 is likelier.