The VESA Mount Adapter is a separate removable component from the rear enclosure, but that part number (923-00557) corresponds to an entire replacement rear housing. I'm not aware of a part number that allows an AASP to order just the VESA Mount to convert an existing rear housing.
That said it is possible, if you swap the VESA Mount, VESA Tongue and the internal mounting plate in place of the stand tilt mechanism on an existing rear housing. It isn't welded into place or anything of the sort, just screwed in. However it does require dismantling the entire computer to get to it which on these models is somewhat of a pain, requiring the renewal of several screen VHB adhesives in the process.