Someone asked me to post pics for discussion. Watch on left is 3500, on right is 3500/3504. Watch on right is 109 earlier than on left. Thanks. 
Left inside caseback:
Right inside caseback:
Left back crown:
Right back crown:
3500 movement:
3500/3504 movement:
I don't know whether the discussion is supposed to be about any specific aspects of either watch but the comparison is interesting, to me, for the reason that the watch with the earlier serial number (the one on the right) appears to have the later case and bracelet, although the earlier dial. Presumably the case (and crown) was changed to a more recent version during a service.
You don't often see two of these rare black Oceans together in one picture. Indeed the differene in colour of the crown could be a replacement during a service.
As I understand it, when previous model cases were/are changed on service (usually because bracelets have to be updated), they were replaced with the then current reference case. So, depending on when the service was carried out, a 3500 case might have been replaced with a 3504 or a 3524 case (the reference 3524 was after 1995).
The watch kept its original case back with its serial number on the outside while the old reference 3500 or 3504 was "dashed" and the new reference was stamped/engraved inside. The references 3500, 3504, 3524 refer to the 1st, 2nd, and 3rd generation bracelet designs respectively.
