Eterna was the first company in the word which has introduced a wrist alarm watch. That happened in 1914 when Etrena transformed their patented (patent 42203) pocket watch movement to wrist size movement. First wrist watches used the same approach to set alarm on/off as pocket watches with the on/off button at the back side of the watch. This was of course very unpractical for wrist watches. By 1938, Eterna made three modifications and then stopped producing alarm movements alltogether. When alarm wrist watches became popular, Eterna used mainly A.Schild movements and has never tried to developed another alarm movement of its own.

I don't have any Eterna alarm wrist watches. I only own one pocket watch.

Instruction manual:

Central crown: Winding time and alarm. Setting the time can be done only by simultaneously pressing the nail button at the right side of the crown. 

Bezel: Setting the alarm hand.

Button at the back plate: Alarm On/Off