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Personally I am convinced Tesla planned the additional part all along.

The signs were clear in all the shipping 5 seaters as well as in the Design Studio almost from the start:

Speculation: Model X five seater is missing a panel to hide seat bottoms?
I'll get you a picture, it is a very different piece/solution than the picture you lightened from the Design Studio. It is a solid 1x2 covered in the interior material, permanently attached in the car but not attached to the seat in anyway. It supports the second cover. Again, I will get you pictures Sunday.
 
Picked up my (our) car this am. There was a permanent support behind the second row, bench seat which block things from sliding under the seat and serves. I had both covers and I am very happy with the space as configured. I only got to drive it for 40 minutes taking it to the detailer for Crystal Serum coating. I did not want to leave it. Amazing car to drive. If that aren't new pictures by Sunday I'll puts some up.

I'm curious, when you leave the car at the detailer, do you set it to the valet mode, like there's been mention elsewhere on the forums?
 
Here's the response from my DS.

I looked at your Monroney and it looks like that’s a line Item left over from we used to sell 85kwh battery cars. When someone “upgraded” to a 90D it would be an included range upgrade. Now that we don’t sell 85kwh cars, every 90kwh car has a “Battery range upgrade” included. I hope this helps and I hope you enjoy your new Tesla!

My New Ride.jpg
 
Headroom in the middle seat is excellent. I am 6'1" and it was fine. There is a 'software' issue with the back seats at the moment when sitting on either side the center seat. An alert is displayed on the dash stating that the seat is 'unsecured'. Tesla told me they are aware of it and to expect an update soon.