camlobe-
The production line for the 707 airframe was still going in the late 1980s, albeit at a low rate, with US Navy E-6s taking up most of the slots. Bearing in mind that most airframes off the line were basically empty, do you mean the 707-to-E-3 gubbins installation line?
I could be wrong, but I believe the last RAF E-3 may have been the last 707 built.