Hi Chris, OG, Brian, others;
A thoroughly worthwhile (continuing!) discussion in my view.
Brian I recall the ventral fin "addition" on the B707 when building models (Revell) of the aircraft as a kid. There was a time when it didn't have it. I think too, it was on the B720. I ran across this comment in Tech Log from "
411A":
Ventral fins were required during US certification to provide greater yaw stability...they came in two sizes, 13 inch and 39 inch, and were eliminated on later aircraft when the vertical stabilizer height was increased.
Note also that all aircraft that had ventral fins installed also had parallel type yaw dampers as standard fit.