It is one of two things and an air refueling track if between 15k and 22k AGL or there are equipment sets that can be strapped to the floor. I worked this airframe for 12 years and one thing was a radio relay equipment for listening because the amount of time this aircraft can stay airborne… Now if this is one of twelve KC-135R that where retrofitted with air refueling systems original fitted on a boneyard decommission B-52, then we can have a bunk set installed and the aircraft could stay airborne in up to 9 days at a time. By having 5 crews rotating bunk time. The max time in limited to wastes removal vs. food/water/ and the next inspection time for engines and other support equipment…