Have you asked the supplier what are these extra pieces for? Could it not be with/without the third seat option version? I did not see it, but I assume there may be a difference with the kinder seat - you do not have the wooden box.
I suggest - when you are fitting the carpet, try on also the covers for B-pillars and make sure it all fits well together.
Also: I do not know if you are replacing just this piece or all the carpets. I do not see the hole for light switch on the KHM set in driver's carpet. In case you need - SLS sell a nice plastic collar to finish this hole (in several colours) - you may want to take a look at it.
Other small items - finish of the gear shift in Manual cars or lack of finishing tape on bottom edges of the sills covers - just maybe not to forget, if you care about those, that is. I wrote that as I see thses items on KHM set...
Differrent topic, sorry, related to what Jordan wrote about the tunnel carpet: I see the rubber (or plastic padding) on the heel rest area on the driver's side carpet. I have seen it on a lot of original carpets. I remember when I was restoring my car it took me a lot of time to find a proper piece that looked like original (size and ribs, certainly without the part number that you would have on original) - I think the availability is better now.
But the tunnel piece: there is also a piece of padding there. I saw it on originals on early cars only. Was it there for late 280SL? I have the impression it was not, but I may be wrong...