Generally correct, Luc Lion, and you have perfectly described the unstable tendencies of early model long body Boeing 707 aircraft.
Now, the 'fix' was to first fit a 39 inch ventral fin, but then the height of the vertical fin was increased, and the ventral fin was eliminated on later aircraft, along with improvements of the yaw damper system.
But therein lies a slight problem.
IF you increase the height (area) of the vertical fin too much, the aircraft may then not meet the certification requirements concerning spiral stability.
Aerodynamics...may times a large tradeoff is made so certification can be completed sucessfully.
I really would like to take a few new generation jet transport pilots (both Commanders and First Officers) along on a flight in one of these really old long body 707's, and watch the astonished looks on their faces, when the yaw damper was switched off, during takeoff and landing, as it was required to be, in conformity with the checklist.
Throw in gusty crosswinds during the landing, and it could well become quite a wild ride.
It surely would be worth the price of admission.