This was The Last Days of Pompeji, a cheap Reader's Digest version of Quo Vadis, officially credited to Mario Bonnard who went ill and Leone took over as director. His unofficial debut, he also was co-screenplay writer.
And you'll find nothing in this simple film which looks like it could have been directed by Leone.
Corbucci made the 2nd unit, with Enzo Barboni (alias E.B. Clucher) as 2nd unit photographer.
Tessari was assistant director and also co-screenplay writer.
Franco Giraldi was also involved.
All 5 were also working on Romulus and Remus (1961), which was directed by Corbucci.
These 5 amigos are responsible for every important early SW up to mid 1966.
Corbucci's story is denied by Leone, who has claimed not to remember it. But even if it's a legend, it's a fitting one.
Print the legend!