One thing that ties Temple XXI to Temple XIX and K'inich Ahkal Mo' Nahb' is those stairs that you might have climbed up in walking through Temple XXI. They are exactly the same construction style as those of Temple XIX. Since the side rooms of Temple XXI had never been explored or the remains of the fallen roof cleared away, there was always a good possibility that some monument connected to Ahkal Mo' Nahb' would be discovered.