Certina 13-20-770, Mainspring

• 
  CERTINA original part in factory packaging.

• 
  Fits cal: 13-20, 13-21

Versions:

• 
  13-20: 19.800 A/h.
• 
  13-21: 19.800 A/h, threads for case clamps.
• 
  13-22: 21.600 A/h, threads for case clamps.