Just to be clear not all Mustangs that came from the factory with 19" wheels have the strut brace or the K brace.
A guy I work with has a GT with the 50th anniversary appearance package. It came with 19" wheels and has neither the K or strut brace. If you order the base Ecoboost with the 19" wheel & stripe package you don't get either.
If you order the 20" wheel package on either the Ecoboost or GT you get the strut tower brace but you don't get the K brace (correct me if I am wrong about the K brace). The Ecoboost with the Performance Package comes with 19" wheels and has the K brace but no strut tower brace. The GT with the Performance Package comes with 19" wheels and has both the strut tower and K braces. The key to getting one or both braces is the inclusion of high performance summer tires. Both Performance Packages and the 20" wheel package come with the summer tires.