"IF" the manufacturer has an "appoved slave program" they can use the same slave part on several aircraft in production. The part is not to leave the production facility and is to be removed prior to any flights after the certification is issued. Someone down the line will get that "new" 50 hour part on their new aircraft, but they should be informed and offered the option of taking it (with some benifit) or opting for a new part.
After certification, and into the kitting phase all those rules go away and you are to the scruples of the manufacturer or finisher. You could get an old seat or old engines installed legally, but you have to accept it. They seem to get you over a barrel when you pay such large amounts of money in advance of ever seeing a logbook or anything but a purchase order. It is hard to walk away from a new ship that you have already paid a million or more dollars into before you see it, knowing the next one available could be months or a year away. That is why we have courts.