>>9700561K2-SO wasn't given any special markings to single himself out as looking any different from other KX droids, there was a scene in the movie where they made a point of drawing attention to that, so if you give a K2 figure an E-22 from a shoretrooper then you have a KX droid from the purge scene. It's free money for hasbro if they release one themselves, since it'd be a straight repack of K2 with a different blaster and maybe some more dirt/dust applied, but if they don't do that it doesn't matter too much.
