I initiated what became a long thread on this on Tanknet about 5- 6 months ago. Some things are clear:
There were at least four FT17s in Afghanistan in the pre Soviet invasion days
These tanks were not (as some sources suggest) captured from the British in the 3rd Afghan War. This arose out of a confusion with the small town of Tank on the N W Frontier that was captured and re captured in this war (this leading to reports such as Afghans capture tank and British recapture tank). There is also a misleading picture on some web sites that claims to be a British WW1 tank taken in Afghanistan, this is in fact a Lloyd Carrier from the late 1930s parked at the foot of the Khyber pass. No tanks were used in this conflict
The most likely souce of these tanks is Iran who apparently purchased a number 2nd hand (and not directly from France) and it is thought contributed some when they intervened in some of the Afghan civil wars of the 1920s