Just to be a nitpicker the C-135 is not a 707 it is 12inches narrower, and also shorter than the 707 though they have the same birth from the Dash80.
If it had newer CFM-56 engines it could have been one of the TACAMOs from here at Tinker or Pax in MD
. If it had the older engines (the TF-33 like we use on the E-3) it could have been the Paul Revere Test bird out of Hanscom/Edwards for our E-8/RC-135 replacement testing.
Engines are a big factor on this one, any ideas. As for the callsign, Harpo i've talked to several diff types on that one, and don't know of any correlation of the top of my head.
Ciao, and Hook 'em Horns,
Lt-AWACS, Yankee Air Pirate