Here is my understanding:
Let's take a VSF employee, lets called her Xi.
Xi arrives at work and gets ready for her shift. She goes over to area where they keep the big boxes of parts and grabs some trays of bits and goes back to her work station.
She got a tray with 100 pips, a tray with 20 cases, a box with 12 S19 movements, a box with 100 dials and a box with 1000's of hands..
Xi builds some watches before lunch. and couple more after. She has run out of cases so she goes to collect some more. While she was away busy, some new cases arrrived, these new ones had S39 on the rehaut. they just got put in the ame box as the S19's. Xi cares not.
Xi grabs some more cases with S39 and continues to make more watches with the parts she has on her desk.
Later, a forumite who does not understand how it works gets all confused becuase he thinks a case with S39 should have the new or old style pip or this or that. But there was never any rule about how case goes with the old style or the new better 3d hands in the first place. Xi was just one of a team of emplyeees throwing whatches together with whatever the boss has put in the parts bins from wherever he has managed to score parts form wherever has not been shut down by the cops.
Maybe im wrong, and the R&D team sat around the board meeting table brainstorming at VSF have strict rules about what parts go with which edition of serial number, and a system like the Porsche factory, with specific parts arriving at each workstation and marrying up with each car on the line on time delivered by robots who keep track on the supply and build process with help from a huge live database computer.. etc.