No idea what the story with Xtra is. What I do know is that Sky King has just relaunched, and supposedly their aircraft is in the new re-branded XTRAIRWAYS livery. I checked Xtra's website, and it lists a fleet of 4, with one based at MIA, which I would assume is the Sky King aircraft. Not sure how many SK has flying yet, or if there is more than one flying for Xtra.
Xtra has the others based at SAT, BWI, and PIT.