For a data point, my car was delivered on Saturday 12/20 without the tan "next gen seats". Production started on 12/13 and was complete on 12/18. My DS informed me of this ahead of time, but when I took delivery the seats were not listed on the "Delivery Declaration and Due Bill" document. I had to get them to add it to the document. So not all cars are coming off the line with next gen seats, its still a mystery as to which car gets them.
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By the way, quality control is a little suspect given the rate they are cranking cars. My car has a carbon fiber "yacht floor", yet I did not order it. Tesla made a mistake in the build, and did not catch it in QA, to my advantage. I pointed it out to the delivery experience specialist (he was a QA engineer helping out on Saturday) but he didn't seem to know what to do at that point. It does make me wonder if there are other things not quite right with the car. . .
Yacht floor is included (unfortunately) with the premium interior now.