Thank you for the eBay link, but that isn't the correct part. That is actually the rear bumper part that all Model S cars have. The reinforcement crossbar actually gets installed underneath the rear bumper part. Remove the plastic fascia, remove the steel rear bumper, install the bumper reinforcement cross bar, reinstall bumper and reinstall fascia.
I cannot find any reinforcement crossbar pieces (PN: 1015811-S0-B) listed on eBay at all, unless I am not searching very well...
Here is a picture of the car with the steel rear bumper removed and the cross member installed from the Service Bulletin:
View attachment 267924