Even though Seiko is now well known to mass population primarely by it's quartz movemetns and watches it was and still is one of the most important producer of the mechanical watch movements. 

Seiko entered into the alarm mechanical movement world in the mid sixties when it's first alarm movement 4005 A was introduced. It is the movement with two spring barrels, watch barrel has an automatic winding (and only automatic, hand winding is not possible!) whereas alarm barrel is hand winded. Tone spring is used for the ringing. For the alarm setting Seiko use very unusual approach, instead of "standard" alarm hand or disc here alarm disc is integrated into bezel. Movement has 27 jewels and has also a date complication. This movement is relatively rare since it was very soon replaced by 4006 A model which also has 27 jewels and in additon to 4005 a day of the week complication is added. The number of jewels were latter reduced to 17, so later 4006 A models have 17 instead of 27 jewels. I have seen also some watches with 21 jewel mark on the dial but I can't confirm that this is correct number in the movement. I have also seen watches where 17j was declared in the dial but on the rotor of the movement 27j was noted.  

All those movements were used only in Seiko watches, all watches with alarm movement are named Bell-matic.

4005 A

4006 A