I'm sure that Fiat never made them, except perhaps a couple of special cars for a Fiera del Levante show in Bari.
Some are later adaptations (don't forget that the Jolly trend was born in the late '50s, when the 500 C was no longer in production) and some are even modern fakes.
Another point is that I never got any information about period conversions (I mean, a period ad or magazine feature). Of course, this doesn't assure that they were never made in the 50s. But, before seeing a period document, I continue to be skeptic...