I spent some time looking for a nice set of early 6139-600X watches and there is certainly a lot of them out there and also a fair number of mix-and-match with parts from lots of individual watches. This got me thinking a bit about the dating clues for these watches. So far I have (and these are just broad guesses without having searched the whole internet) Feel free to correct as this could provide some guidance for others:
Two piece second hand: Good dating evidence, not much overlap, switch Q1-Q2 1970.
Notched case: Good dating evidence, not much overlap, switch Q2-Q4 1970.
Flat hour and minute hands:Good dating evidence, not much overlap, switch Q1-Q2 1970.
17/21j 6139A caliber: Not dating evidence, complete overlap.
6139A/6139B caliber: Fairly good dating evidence, some overlap, switch
Proof/resist: Fairly uncertain dating evidence, overlap large between geographical markets, resist starts Q3 1969, proof ends ?.
This is what I ended up with: a yellow dial JDM 6000 from June 1969 and a blue dial 6009 from November 1969.
- martin
Two piece second hand: Good dating evidence, not much overlap, switch Q1-Q2 1970.
Notched case: Good dating evidence, not much overlap, switch Q2-Q4 1970.
Flat hour and minute hands:Good dating evidence, not much overlap, switch Q1-Q2 1970.
17/21j 6139A caliber: Not dating evidence, complete overlap.
6139A/6139B caliber: Fairly good dating evidence, some overlap, switch
Proof/resist: Fairly uncertain dating evidence, overlap large between geographical markets, resist starts Q3 1969, proof ends ?.
This is what I ended up with: a yellow dial JDM 6000 from June 1969 and a blue dial 6009 from November 1969.

- martin