Seiko Spirit (SCVS001 vs SCVS003)

6R15 的機芯有 3 種, 6R15A、6R15B 同 6R15C

Late 2005 – 6R15A introduced, based on the 7S with Etachron regulator and Spron 510 mainspring. 

Late 2006 – 6R15B introduced – new parts: 
- the barrel and train wheel bridge 
- the movement barrel (complete) 
- the centre wheel and pinion 
- the ratchet wheel the oscillating weight
- the ratchet wheel screw 

Late 2011 – 6R15C introduced – changes: 
- modified ratchet sliding spring
- modified date indicator maintenance plate 
- new balance wheel 
- new barrel 

Original Article: http://watcharama.com/the-seiko-4r36-family-in-brief/

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