Glad to be of help, I don't think it is a Frog kit, as they did the Macchi 202 Folgore which whilest very simular was a totally different bird. As far as my memoury goes the Fiat G 55 was first issued by Aliplast then by Supermodel when the partners of Aliplast seperated one forming Italeri the other forming Supermodel which were eventually absorbed into Italeri. That's when they re-issued the G 55 Centauro.
Maybe Frog did a licenced issue of it in the late 60's or early 70's I cann't comment as I was out of modelling then, like being a life-guard in Newquay, so maybe Frog did issue the kit.
The Macchi 202 was powered by a Daimler-Benz 601A engine and was armed with machine guns while the Fiat G55 used the DB 605A engine and mounted cannons in the wing, that's what really makes it a G55.
Remember we do this for fun John the Pom