The Boeing 707 ventral fin story
Here's another one for you wise men.
I believe some European customers for the 707 insisted on a ventral fin (VF) for certification purposes ( BOAC, Luft ) but was, I believe, adopted by US operators as well 300/400 ? series for better yaw performance. But not all US operators used it.
However the VF was not universally used and later 707 production did not use it. Was this to do with flight control improvements or euro regulators being overly demanding and it wasn't really that esential?