Squib66,
All Rivet Joint airframe and mission systems modifications are overseen by L-3 Communications (previously Raytheon) under the oversight of USAF Materiel Command.
Conversion of the former KC-135Rs to RC-135W standard for the RAF was of course done there.
RC-135V/W Rivet Joint > U.S. Air Force > Fact Sheet Display
http://www.raf.mod.uk/equipment/rc135wrivetjoint.cfm