I once had a cheap Brazilian made ~.41 caliber replica smoothbore that had a similar full stock configuration.
That small gauge gun was said to be a parrot gun for shooting smaller tropical birds and mammals in the jungle environment of South America.
They even shoot small monkeys in South America because it's difficult to preserve meat in such hot climates and a little fresh meat is better than none. 12mm equals .47 inches.
A large gun isn't really needed for small game and these small bores are economical to load. Whether it was intended for export or for use in Europe, other European countries shoot song birds for sport and as a food source including Italy. Just look at how small some of our game birds are like dove and some quail.