As with most things, there are few absolutes. A lot of info in literature has not been accurately researched or flat out wrong information is repeated over and over.
The transition from the full 13 digit VIN stamping to a 8 digit VIN most certainly did happen in January of 1969. It can be shown that cars with a 13 digit stamping and a mid January SPD exist (see below).
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Meaning... it is unlikely that the cut over was a hard and fast 1/1/69. The transition likely took place over time at the various plants.
This is another reason why factory documents and tags are important to save as well as documentation on cars that are parted out. Many changes to details took place over a model year. Documenting those changes is important.
