Thanks!
According to Alessandro Sannia's encyclopaedia F.lli Zanella was founded in 1955 by brothers Luigi, Gino, Guido and Giuseppe Zanella to carry out body repairs.
So it seems they were around a bit earlier than that, even carrying out coachbuilding, but it's possible that the formal company was only established in 1955, hence the confusion around the date.
Sadly the cost of converting the company into modern coachbuilders in the 1960s led to the closure of the Carrozzeria in 1970.