The second row backing is definitely not a one off. Just went to look at one. Indeed, the back of the second row is made out of some interesting material, not very Tesla-like. It looks like trunk liner to me. Which is great if you plan on folding them a lot, but not if you have passengers back in the 3rd row quite often. I wish they still offered the monopost option as well, even though I like the fact that now the seats can fold flat if you need them to.
Now, the reason I actually hopped on the forum was to figure out what the zippers do. I was quite curious about that. I noticed them, but they seem to run only on the inside of the seat, where the split happens. At first I thought it's so you can take off the covers and wash them easily. Doesn't seem that's the case. I wish I took photos, but I was in a crazy rush, only had about 10 minutes to look at the back seats (drove almost 2 hours just to find a 7-seater in stock somewhere that I can check out).
Unfortunately, it looks like in the configuration I'm trying to get, they are out of "new inventory" that has the mono post seats, so I'll be stuck with the manual folding ones, if I decide to go with the 7-seater.