Hi!
I flew the KC-135R. In learning to fly the -135, I found out it was the next model in the series after the -707, and if it was a civilian plane, it would've been called the -717. That is why the Boeings went from a -707 to a -727, "skipping" the -717 designation. The "skip" was the -135.
I have been wondering the same thing about ads asking for currency in older jets, such as the -707, -727, DC-10, L-1011, etc.
cliff
KMRC (today, based KYIP)
DC-9 Captain
USA Jet Airlines