This is based off some thing I have seen before... It could be an attachment used to test de-icing on new aircraft, they fill the tanker with water and spray it on the test aircraft to test its de-icing systems... Thats the only thing that comes to mind.
USS Harry S. Truman, where having fun not only happens, it's also mandatory.
That fitting on the boom was used to spray out water. That particular KC-135 was used during FAA icing tests as part of the investigation of American Eagle flight 4184, an ATR-72 that was flying in icing conditions and crashed on October 31, 1994 near Roselawn, Indiana. FAA investigators suspected that supercooled rain drops created an ice ridge behind the deicing boots, which disturbed the air flow enough to where the aileron control was ineffective. The FAA used an American Eagle ATR-72 and flew it behind the KC-135 as it sprayed a yellow dyed water mixture, which would show the areas of ice build up. Hope that helps.