Police in inter-war Germany used a variety of submachine guns, and sometimes added a distinctive extra safety mechanism to them. No patent or documentation ha been uncovered (that I am aware of, anyway), but the exact same device is found on gun from the MP18,I through the MP40, including the MP35, MP28,II, and Erma EMPs like this one.

The device is simply a rotating locking bar that engages into a slot cut in the bolt of the gun. When engaged, it locks the bolt in the forward position. Installation was simple, requiring a slot in the receiver and bolt and three tapped holes by which the safety is screwed onto the receiver. In some cases (like the MP40) a slight relief of a stock or handguard was also required to fit. Not all police SMGs have this device, but it is not an uncommon addition to find.