Hi TheFinn

"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."

These B707 were delivered to Qantas with Cargo doors fitted and it was part of the pre-service mods to bring them up to full Cargo/Conversion standard.
Qantas aircraft were generally called "convertibles"
The change over from pax to cargo was not quick and entailed fitting up interior panels and floor roller system, not QC as in latter day aircraft.
Generally change was done at maintenance  time.

Regards
Glenn