That jack is very different from Italian flag:
This flag contains four quarters, each one representing, respectively, one of the main Italian "maritime republics" or thalassocracies (from Greek, "rule of the sea"): Venice, Genoa, Amalfi and Pisa.
- Venice is represented by the Lion of St. Mark (symbolism from Revelation 4:7), because it's believed that the body of the evangelist rests in the cathedral of that city.
- Genoa was the first of many Northern Italian cities to adopt the St. George's cross, but probably there isn't connections between that and the St. George, even though he's patron saint of Genoa.
- The Amalfi's is very similar to Maltese cross (but with blue background instead of red), and it's possible that the former predates the latter: merchants from Amalfi founded the hospital where the Order of St. John (Knights Hospitaller) was based.
- The cross of Pisa was granted by pope Benedict VIII to the fight against Saracens in Sardinia.
While the anverse of the flag shows St. Mark's lion's head, the reverse shows his tail...
Additionally, the merchant vessels politely flies a jack without the sword. The open book reads "Pax tibi Marce, evangelista meus" ("May peace be with you, Mark, my evangelist"), what, following the tradition, an angel said to Mark when pointing his burial place at Venice.
Comments and suggestions are welcome.