A photo or two would be helpful.
From your description, I am led to believe that your find may possibly be a short-barrel shotgun, illegal without an accompanying special federal tax stamp (among other things).
• Is the attic you mention your own, or someone else's?
• Are the two barrels smooth inside, or do they have internal grooving (rifling)? (If they're grooved, you're OK—it's just a pistol.)
I believe that Aco Fino may mean something like "high-quality steel," since "steel" in Spanish is acero, and in Italian accaiao. Fino means "fine" or "high quality" in both languages. (Aco Fino might also be Portugese, or Brazilian.)
The "450" is probably just the inside diameter of the barrels: 0.45", or .45 caliber. However, it might also be a model number.
If you can make photos, please include all of the markings and stampings that you find underneath the two barrels.
If my conjecture is correct, the barrels come off (to make photography easier) by first removing the fore-end (the wood underneath the barrels).