Your Luger has been fitted with a "magazine safety", or as policeluger identified it, a "trigger safety". The purpose of this device was to prevent the weapon from being fired by blocking the trigger from being pulled when the magazine was removed but a live round was in the chamber. A totally cumbersome concept that was quickly abandoned. Nearly every Luger so equipped was modified by cutting off the portion of the device that went behind the trigger to block it when the magazine was removed, as policeluger explained, or by removing the device entirely. Here is a picture of both safety devices.
