These heavy (more than 500 grams) handcuffs were made by the famous Horst Stein company of Munich, Germany. They belong to the Horst MOABIT series of restraints and is Type 616 with swivel. The lock-cases and stationary bows are made of aluminum. The rest is made of steel. To open the cuff from the single lock position the key has to be turned about 40 degrees counter-clockwise. Turning it 90 degrees clockwise will double lock the cuffs.
Horst Stein produced handcuffs from the 1940ies until the 1970ies. The company usually had only very few items in stock, which makes the MOABIT handcuffs a rare and highly sought collector's item.