Pair of 15" guns. Note that the turret is open, as well; there's a big gap between the turret sides and the "roof" (which is more of an umbrella, really....) She drew 3000 tons, and had 465 horsepower of propulsion, so speed 1 in flat seas, 0 otherwise.... She also had a sister, the Alfredo Cappellini.
Note that the Cappellini was sunk in 1917, while Faa Di Bruno was reclassified, becoming floating battery GM194 - Faa Di Bruno in WW2 was a submarine sunk off Ireland in 1940.
Dom.
PS - DM - you're on the money, I think; Venetian lagoons and the Adriatic on a calm day were about it all it was designed for.