The Cannone da 90/53 was an Italian-designed cannon used both in an anti-aircraft role and as an anti-tank gun during World War II. It was one of the most successful anti-aircraft guns to see service during the conflict. The naval version of the gun was mounted on Italian battleships of the Littorio-class and the Andrea Doria-class.
The designation "90/53" meant that the gun had a 90 mm caliber and a barrel 53 caliber-lengths long.
Learn More on Wikipedia