I don't think anyone has ever suggested that all taxis without exception were 217s. But the 217 was a model built for high-mileage, high-usage applications like company cars and taxis. So they were popular with the fleet taxi companies, but obviously not popular enough as they were discontinued in 1958. (FC-217s are very rare, almost as rare as FE-219s)
I agree with Rob that these taxis are probably privately owned cabs. If it was my cab I'd fit the light to the sun-visor too! Much cheaper to replace a sunvisor than get holes in the roof fixed
Remember that in the late 50s and early 60s the fitting of two-way radios to taxis was not that common, so the benefit of belonging to Premier Cabs or St George or De Luxe or Silver Top or whoever was not so great. Most cabbies had to make a living on getting flagged on the street or waiting at the rank. Geoff (Gertie) worked as a cabbie in the mid sixties, he might be able to add some comments.
As an aside, it has been overshadowed by the 50th Anniversary of the FJ, but this year is actually the 50th Anniversary of the Business Sedan, which was introduced in the last few months of 48 production before the release of the FJ. The 217 lasted about 5 years.
cheers
RET