I do safety certification for a major OEM and use to work for Chrysler back in the day. What is being referred to as the the mylar is the safety certification label (already discussed) that is required by the Code of Federal Regulations (CFR) to be applied to a vehicle as it completes its production and certifies that the car meets all safety regulations in effect on the day of production and can be sold to the public. Once a vehicle is certified, it can not be uncertified. The month and year of production must be the month and year shown on the label, but mistakes can and are occasionally made and since this label is not part of the Federal Motor Vehicle Safety Standards (FMVSS) (Part 571 of the CFR), it is not recallable if incorrect and would be considered an inconsequential non compliance if the date was wrong because someone in the factory did not update the software or typeset. That does not preclude a vehicle from being certified. Also, since the label is typically put on on the final assembly line, the vehicle still has some "build" to go through, which includes the rolls test, usually one of the last things before the vehicle is shipped, If something goes wrong on the rolls, such as they blow they engine, it would go to the repair hole, where it may sit for a while until there is time to work on it, especially if it requires an engine change. If it was an engine change at the factory, they would restamp the VIN on the block before they shipped it, as that is a anti theft requirement. So it is feasible that a vehicle has a March build date and an April engine if 1) the Cert Label was incorrectly printed or 2) there was an engine failure after the cert label was applied and the engine was replaced in the factory because the cert label would not be updated in this situation because you can not uncertify and re-certify a vehicle.
I did not read the entire post, so I hope this point has not already been discussed.
My 2-cents.