Certainly could be a STP-2 booster! Evidently 2 side boosters from the Arabsat-6 FH will go to STP-2.
This has got to be a weird situation for USAF, doing the paperwork for a cert flight with re-flown rockets.
For the sake of the STP-2 schedule, I hope the return to launch site goes well. Not sure how long it will take SpaceX to come up with another 18 Merlins in FH form.