It wasn't even a 707, it was a prototype model 367-80.
There were some unusually significant structural differences between the prototype dash 80 and production 707's, for example:
> The 132 in (3,400 mm) wide fuselage of the Dash 80 was large enough for four-abreast (two-plus-two) seating like the Stratocruiser. Answering customers' demands and under Douglas competition, Boeing soon realized this would not provide a viable payload, so it widened the fuselage to 144 in (3,660 mm) to allow five-abreast seating and use of the KC-135's tooling.
There were some unusually significant structural differences between the prototype dash 80 and production 707's, for example:
> The 132 in (3,400 mm) wide fuselage of the Dash 80 was large enough for four-abreast (two-plus-two) seating like the Stratocruiser. Answering customers' demands and under Douglas competition, Boeing soon realized this would not provide a viable payload, so it widened the fuselage to 144 in (3,660 mm) to allow five-abreast seating and use of the KC-135's tooling.
https://en.wikipedia.org/wiki/Boeing_367-80
https://en.wikipedia.org/wiki/Boeing_707