I have 2 bench seats from 1970 Buicks. The pearl white bench has a CLEAR stamped date code of 6-16-69. Assuming this was a "leftover" from a 1969 with pearl interior. The other headrest is from a green 1970 bench, just basically letter date codes... Enjoy!
Who knows when they started making parts for the next model year? Production started in August so they had to have a good supply of parts on hand.
They started phasing in parts for the upcoming year around May/June. I’ve had a number of very early cars that had May parts on them.