No, I don't agree with you. I would exclude that Fiat ever made such stange bodies during the Fifties. At that time the policy was to outsurce every "non-standard" coachbuilding and also version much more diffused were actually built out of Fiat, sometimes with the badge of the coachbuilder and sometimes not.
A few examples: 2100 and 2300 Berlina Speciale were made by Ellena; 1800, 2100 and 2300 Familiare were made by Savio; 1400 Cabriolet was made by Colli; 1300 and 1500 Familiare were made by OSI.
I can't imagine a 1100 Taxi made in Mirafiori with such a body!
Let's discuss the other guesses.
Michelotti at that time was ony a designer. It had a little workshop for prototyping about ten years later, but never made serial production (except the "Shelette" beach-car). On the other hand, probably he was the actual designer of these cars, at least because it was the designer of a half of that period's Italian cars...
Frua at that time had a little workshop, but nothing similar is featured in the very comprehensive research of Dr. Dierkes (see his website), that's based on Pietro Frua's archive.
Lombardi and Vignale could theoretically be. I would say the first one.
Here are two drawings, one from the Michelotti's archive and another from a period advertising: they look like the taxi posted by you.
But the mistery on my one is still there...