S211 (wagon) Factory trailer hitch in the US
 

So the S211 factory towbar was never offered in the USA, but I'll be darned if that is going to let me take a Mercedes to U-Haul for an aftermarket installation or try to do something myself involving drilling into the car.

In case you have this same inkling, the two important part numbers are:
2113100104 - Tow bar
2115450832 - Control unit

The control unit was easy enough to find on eBay shipping out of Germany (though a surprising number of other MB Parts do not recognize the number at all), however the tow bar proved to be more of a challenge. I could find a couple used units on eBay.de and one in Australia, but nobody who had them was willing to ship to the USA. Mercedes-Benz blocks shipments of parts to the USA if they were not originally offered in the USA market. Many sites list the part as available along with a price, but if you try to order or call and ask first, they'll say they can't ship it to the USA. I was able to determine there were three new ones in Germany, but no idea how to get them. I briefly considered having relatives in the UK order it for me, then ship it here... Even the used tow bars are rather expensive though, and since I knew the new part existed still I wanted to find a way to get it.

Anyways, in case anybody else is on this same Unicorn Quest, I finally found a place to buy the tow bar new from, shipped from Germany for a rather amusing figure of $13 shipping cost. Before you get your hopes up too much though, the price of the part is $1343. Still undaunted? There should be two more sitting in Germany waiting, and you can order it via this website:

https://is.gd/Y4hsEx (short URL used because PeachParts is blocking ECS Tuning URL's for some reason)

Hope this helps someone... If anyone has added a factory tow bar to an S211, I'd love to hear about any other concerns I should think about!