This may be old-hat to the afficionados, but I've found a well-researched article on the 'net, originally published in a magazine called "Airways" (?) in 2010, here courtesy of the Renton (Washington, US) website:
http://rentonwa.gov/uploadedFiles/Li...20combined.pdf
Compare the fins (VSs) on the photos on pages 32 & 33. The account of the British certification process that led to the modifications starts on the right column of page 35.
I hadn't realised that the ventral fin retrofitted to those early models was dual-purpose - it also stopped the a/c over-rotating on take-off, in the absence of a suitable stick-shaker for that purpose. ** The other part of the B707 "fix", the top extension, is clearly visible when you compare the two photos.
The B707-320B/C "Advanced" types, such as I flew in Caledonian/BCAL, had dispensed with the ventral fin, apparently for the reasons stated on page 36 (yaw-damper and a stick-shaker).
OFF-TOPIC
Although the stick-shaker was supposed to prevent over-rotation on T/O, I once over-rotated (and probably rotated at too-high a rate) on a MTOW departure at LAX, and had to check forward slightly before the a/c would unstick.
** The British certification authority, the ARB, had pioneered the test of taking-off with the tail scraping along the runway, and the concept of Vmu (minimum unstick speed), as a result of accidents on the Comet 1.