Hagenuk stands for Hanseatische Apparatebouw Gesellschaft Neufeldt und Kuhnke (or in English something like Machine building company Neufeldt and Kuhnke). The former Ludwig von Bremen company.

The company was based in Kiel Germany and became famous in the early 1920-ies for developing one of the earliest armored diving suits. Visit that section to learn all about that. What many people do not know is that they also built standard diving equipment. However, these helmets again looked very similar to the French helmets.

Hagenuk armored suit 1913

First style of Hagenuk helmet with the square breastplate. Rear view showing the non-return valve on the air inlet elbow.
There is no connection for a reserve air supply The large blanking nut protects the communication fitting.
The helmet volume is much smaller than the later design. There are no weight hangars on the rear of the helmet.
Side view showing the exhaust valve. Traditional German style handle which was fitted to the later style,

Photo courtesy of Jean-Patrick Paszula.

This is the classic 3 bolt design favored by the German, Russian and French manufacturers. The helmet has four windows including a screw in face port.
The connection on the diver's right hand side is for the small breast weight which contains two small air bottles for an emergency ascent and also acts as the front weight. The weight loops are fitted to the breastplate on the front and the back.