The SDK was a silenced bolt-action rifle issued to the Gestapo during WW2. Its possible this rifle inspired the .45 ACP DeLisle used by the British SOE. It was also known as the 'Hitler assisination rifle' and the ammunition issued were totally unmarked 9x19mm rounds that supposedly held cyanide within the soft lead bullet. The silencer is one of the most novel and supposedly is very quiet. With the bolt action allowing little gas leak, it somewhat sounds like a billiard ball hitting soft dirt.
According to Atwood, the weapons were made in 1939, at the direction of the chief of Berlin Police, Graf von Heldorf, who was executed in 1944 for his role in the Hitler assassination plot. At the time of writing of this article, the weapon was still in the hands of the Army officer who acquired it in Berlin shortly after the war's end. Supposedly, its location was unknown.
Back in the late 1970s, the SDK was in several collector magazines claiming only a few of these rifles exist today.