The Gruen "Duotone" uses the wristalarm calibre Pierce 135, which is named Gruen 920ss. There is the interesting feature, to adjust the loudness of the alarm tone with the lower crown (not noticeable by the showed watches). The wristalarm from Pierce has the name "Duofon".
The movement has 21 Jewels and 18000 A/h, 2 springs. Operation manual: Crown a 3 o'clock in position "0" = winding of time (turning to 12) and of alarm (turning to 6) Crown a 3 o'clock in position "1" = setting of the alarmtime counterclockwise Crown a 3 o'clock in position "2" = setting of time Crown a 4 o'clock in position "0" = little turning to 12 = loud alarm tone -- alarm is deactivated Crown a 4 o'clock in position "0" = little turning to 6 = low alarm tone -- alarm is deactivated Crown a 4 o'clock in position "1" = little turning to 12 = loud alarm tone -- alarm is activated Crown a 4 o'clock in position "1" = little turning to 6 = low alarm tone -- alarm is activated
Pierce developed it's alarm movement in late fifties. It was called Pierce 135 and it's even today among one of the most interesting alarm movements. Unfortunately Pierce dissapeared during the quartz crisis.
Pierce 135, also known as the Pierce Duofon is known to be the only alarm movement which has two phase setting for the alarm loudness ringing. At the time when most mechanical alarms were developed, two totaly different directions were followed. First - alarm wrist watch is alarm clock which wake you up in the morning. The second - alarm wrist watch is a reminder watch which with it's subtile sound reminds you for the new appointment. Of course, those two directions asked two very different ringing powers and loudiness of the alarm. Well, Pierce solved this equation by introducing two step of laudiness in it's movement. That was done with simple knob which allowed hammer to strike just to a pin (silent buzzing) or also to the tone spring so buzzing and ringing simultaneusly.
Pierce 135 is again the movement with two spring barrels but one stem and crown. Primarely movement was mounted only to Pierce watches (with the name Duofon, Duoton or Duovox), from other companies the most known one that used this movement is Gruen, which called it's movement Gruen 920.