Singapore Airlines also had them:
http://www.airliners.net/photo/Singapore-Airlines-Cargo/Boeing-707-338C/1052602/L/

Think some of them were already cargo-converted at Qantas. And I think, it IS much cheaper to add the cargo-door only.


Dont know if the 707 had such versions, but some other airplanes were or still are "convertibles". They had passenger windows and a cargo-door, so the airline could convert theyr plane from cargo to pax and vice-versa in just a few hours.