The ventral fin was fitted when the B707-100 had the fan engines fitted to make them B707-120 series. They also made the aircraft "geometry limited" as a bonus reducing some margins for the Vr and Vlo speeds. It had nothing to do with the British ARB. There are many other sins to be laid at their door but not this.
Story more complex than that I think. There's a thread on subject here:
http://www.pprune.org/tech-log/36273...fin-story.html
There are pictures on net of 320 series aircraft, including the P&W fan engined 320B series, both with and without the ventral fin. Wikipedia is not a source but the pictures there, including one of last commercially operated 707, illustrate the point pretty well.