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 light relief of a stock or handguard was also required to fit. Not all police SMGs have this device, but it I not an uncommon addition to find.
The safety was called the ‘Blocksicherung’ and it was introduced in 1940/41 for submachine guns in use with the SS and German police.
Indeed, it is almost ubiquitous on German-made EMP submachine guns. In fact it is one of the major indicators as to whether an EMP is a German original or a Spanish copy – none of the Spanish-made guns had this. In fact, because of this, the Spanish had to develop their own mechanical safety in 1944, which took the form of a plunger located underneath the receiver which blocked the bolt path when it was raised. The EMP submachine gun had a poor safety record in Spain ever since Buenaventura Durruti was killed by his own EMP accidently discharging after he dropped it.
Probably most “genuine German” EMPs that go for auction today are actually just Spanish copies made in Republican, and later Francoist, state factories from about 1937 – 1944. For a real German gun, always look for the police safety, a flip-up rear sight, and (most importantly) Erma-Werke factory markings. The Spanish guns lack all three of these features.
I’m not sure I’ve ever seen a Bergmann MP 35/I with this device installed, though such a thing may exist. It was only added to MP 28,IIs and MP 40s on a limited, experimental basis, since those guns already had mechanical safeties of their own. MP 18,Is converted to the “System Schmeisser” box magazine feed are also very commonly seen with these safety devices, whereas military MP 18,Is with their original TM 08 feeds almost never have a mechanical safety.